xfs: Support setting FMODE_CAN_ATOMIC_WRITE

Set FMODE_CAN_ATOMIC_WRITE flag if we can atomic write for that inode.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Tested-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>	 #On ppc64