xfs_io: add MAP_SYNC support to mmap()

xfs_io: add MAP_SYNC support to mmap()

Add support for a new -S flag to xfs_io's mmap command.  This opens the
mapping with the (MAP_SYNC | MAP_SHARED_VALIDATE) flags instead of the
standard MAP_SHARED flag.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Suggested-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>