Add ZFS specific mmap() checks

Under Linux the VFS handles virtually all of the mmap() access
checks. Filesystem specific checks are left to be handled in
the .mmap() hook and normally there arn't any.

However, ZFS provides a few attributes which can influence the
mmap behavior and should be honored. Note, currently the code
to modify these attributes has not been implemented under Linux.

• ZFS_IMMUTABLE | ZFS_READONLY | ZFS_APPENDONLY: when any of these attributes are set a file may not be mmaped with write access.

• ZFS_AV_QUARANTINED: when set a file file may not be mmaped with read or exec access.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>