MFC 294366:
Initialize vm_page_prot to VM_MEMATTR_DEFAULT instead of 0.

If a driver's Linux mmap callback passed vm_page_prot through unchanged,
then linux_dev_mmap_single() would try to apply whatever VM_MEMATTR_xxx
value 0 is to the mapping. On x86, VM_MEMATTR_DEFAULT is the PAT value
for write-back (WB) which is 6, while 0 maps to the PAT value for
uncacheable (UC). Thus, any mmap request that did not explicitly set
page_prot was tried to map memory as UC triggering the warning in
sg_pager_getpages().

Sponsored by: Chelsio Communications