VMCI: check context->notify_page after call to get_user_pages_fast() to avoid GPF

The call to get_user_pages_fast() in vmci_host_setup_notify() can return
NULL context->notify_page causing a GPF. To avoid GPF check if
context->notify_page == NULL and return error if so.

general protection fault, probably for non-canonical address
    0xe0009d1000000060: 0000 [#1] PREEMPT SMP KASAN NOPTI
KASAN: maybe wild-memory-access in range [0x0005088000000300-
    0x0005088000000307]
CPU: 2 PID: 26180 Comm: repro_34802241 Not tainted 6.1.0-rc4 #1
Hardware name: Red Hat KVM, BIOS 1.15.0-2.module+el8.6.0 04/01/2014
RIP: 0010:vmci_ctx_check_signal_notify+0x91/0xe0
Call Trace:
 <TASK>
 vmci_host_unlocked_ioctl+0x362/0x1f40
 __x64_sys_ioctl+0x1a1/0x230
 do_syscall_64+0x3a/0x90
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

Fixes: a1d88436 ("VMCI: Fix two UVA mapping bugs")
Reported-by: syzkaller <syzkaller@googlegroups.com>
Signed-off-by: George Kennedy <george.kennedy@oracle.com>
Reviewed-by: Vishnu Dasa <vdasa@vmware.com>
Link: https://lore.kernel.org/r/1669666705-24012-1-git-send-email-george.kennedy@oracle.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/vmw_vmci/vmci_host.c

@@ -242,6 +242,8 @@ static int vmci_host_setup_notify(struct vmci_ctx *context,
 		context->notify_page = NULL;
 		return VMCI_ERROR_GENERIC;
 	}
+	if (context->notify_page == NULL)
+		return VMCI_ERROR_UNAVAILABLE;
 
 	/*
 	 * Map the locked page and set up notify pointer.