munmap done by iscsiuio during a stop of the service triggers a "bad
pte" warning sometimes. munmap kernel path goes through the mmapped
pages and has a validation check for mapcount (in struct page) to be
zero or above. kzalloc, which we had used to allocate udev->ctrl, uses
slab allocations, which re-uses mapcount (union) for other purposes that
can make the mapcount look negative. Avoid all these trouble by invoking
one of the __get_free_pages wrappers to be used instead of kzalloc for
udev->ctrl.

 BUG: Bad page map in process iscsiuio  pte:80000000aa624067 pmd:3e6777067
 page:ffffea0002a98900 count:2 mapcount:-2143289280
     mapping: (null) index:0xffff8800aa624e00
 page flags: 0x10075d00000090(dirty|slab)
 page dumped because: bad pte
 addr:00007fcba70a3000 vm_flags:0c0400fb anon_vma: (null)
     mapping:ffff8803edf66e90 index:0

 Call Trace:
     dump_stack+0x19/0x1b
     print_bad_pte+0x1af/0x250
     unmap_page_range+0x7a7/0x8a0
     unmap_single_vma+0x81/0xf0
     unmap_vmas+0x49/0x90
     unmap_region+0xbe/0x140
     ? vma_rb_erase+0x121/0x220
     do_munmap+0x245/0x420
     vm_munmap+0x41/0x60
     SyS_munmap+0x22/0x30
     tracesys+0xdd/0xe2
Signed-off-by: Arun Easi <arun.easi@cavium.com>
Signed-off-by: Manish Rangankar <manish.rangankar@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>