Make sure the "vm_flags" and "vm_page_prot" fields get set correctly
in the VM area structure in the LinuxKPI when doing mmap() and that
unsupported bits are masked away.

While at it fix some redundant use of parenthesing inside some related
macros.

Found by: KrishnamRaju ErapaRaju <Krishna2@chelsio.com>
MFC after: 1 week
Sponsored by: Mellanox Technologies