Currently, there's no check if an invalid buffer range
is passed. However, while testing DVB memory mapped apps,
I got this:

   videobuf2_core: VB: num_buffers -2143943680, buffer 33, index -2143943647
   unable to handle kernel paging request at ffff888b773c0890
   IP: __vb2_queue_alloc+0x134/0x4e0 [videobuf2_core]
   PGD 4142c7067 P4D 4142c7067 PUD 0
   Oops: 0002 [#1] SMP
   Modules linked in: xt_CHECKSUM iptable_mangle ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat nf_nat_ipv4 nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 xt_conntrack nf_conntrack tun bridge stp llc ebtable_filter ebtables ip6table_filter ip6_tables bluetooth rfkill ecdh_generic binfmt_misc rc_dvbsky sp2 ts2020 intel_rapl x86_pkg_temp_thermal dvb_usb_dvbsky intel_powerclamp dvb_usb_v2 coretemp m88ds3103 kvm_intel i2c_mux dvb_core snd_hda_codec_hdmi crct10dif_pclmul crc32_pclmul videobuf2_vmalloc videobuf2_memops snd_hda_intel ghash_clmulni_intel videobuf2_core snd_hda_codec rc_core mei_me intel_cstate snd_hwdep snd_hda_core videodev intel_uncore snd_pcm mei media tpm_tis tpm_tis_core intel_rapl_perf tpm snd_timer lpc_ich snd soundcore kvm irqbypass libcrc32c i915 i2c_algo_bit drm_kms_helper
   e1000e ptp drm crc32c_intel video pps_core
   CPU: 3 PID: 1776 Comm: dvbv5-zap Not tainted 4.14.0+ #78
   Hardware name:                  /NUC5i7RYB, BIOS RYBDWi35.86A.0364.2017.0511.0949 05/11/2017
   task: ffff88877c73bc80 task.stack: ffffb7c402418000
   RIP: 0010:__vb2_queue_alloc+0x134/0x4e0 [videobuf2_core]
   RSP: 0018:ffffb7c40241bc60 EFLAGS: 00010246
   RAX: 0000000080360421 RBX: 0000000000000021 RCX: 000000000000000a
   RDX: ffffb7c40241bcf4 RSI: ffff888780362c60 RDI: ffff888796d8e130
   RBP: ffffb7c40241bcc8 R08: 0000000000000316 R09: 0000000000000004
   R10: ffff888780362c00 R11: 0000000000000001 R12: 000000000002f000
   R13: ffff8887758be700 R14: 0000000000021000 R15: 0000000000000001
   FS:  00007f2849024740(0000) GS:ffff888796d80000(0000) knlGS:0000000000000000
   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
   CR2: ffff888b773c0890 CR3: 000000043beb2005 CR4: 00000000003606e0
   Call Trace:
    vb2_core_reqbufs+0x226/0x420 [videobuf2_core]
    dvb_vb2_reqbufs+0x2d/0xc0 [dvb_core]
    dvb_dvr_do_ioctl+0x98/0x1d0 [dvb_core]
    dvb_usercopy+0x53/0x1b0 [dvb_core]
    ? dvb_demux_ioctl+0x20/0x20 [dvb_core]
    ? tty_ldisc_deref+0x16/0x20
    ? tty_write+0x1f9/0x310
    ? process_echoes+0x70/0x70
    dvb_dvr_ioctl+0x15/0x20 [dvb_core]
    do_vfs_ioctl+0xa5/0x600
    SyS_ioctl+0x79/0x90
    entry_SYSCALL_64_fastpath+0x1a/0xa5
   RIP: 0033:0x7f28486f7ea7
   RSP: 002b:00007ffc13b2db18 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
   RAX: ffffffffffffffda RBX: 000055b10fc06130 RCX: 00007f28486f7ea7
   RDX: 00007ffc13b2db48 RSI: 00000000c0086f3c RDI: 0000000000000007
   RBP: 0000000000000203 R08: 000055b10df1e02c R09: 000000000000002e
   R10: 0036b42415108357 R11: 0000000000000246 R12: 0000000000000000
   R13: 00007f2849062f60 R14: 00000000000001f1 R15: 00007ffc13b2da54
   Code: 74 0a 60 8b 0a 48 83 c0 30 48 83 c2 04 89 48 d0 89 48 d4 48 39 f0 75 eb 41 8b 42 08 83 7d d4 01 41 c7 82 ec 01 00 00 ff ff ff ff <4d> 89 94 c5 88 00 00 00 74 14 83 c3 01 41 39 dc 0f 85 f1 fe ff
   RIP: __vb2_queue_alloc+0x134/0x4e0 [videobuf2_core] RSP: ffffb7c40241bc60
   CR2: ffff888b773c0890

So, add a sanity check in order to prevent going past array.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>