ext4: Support setting FMODE_CAN_ATOMIC_WRITE

FS needs to add the fmode capability in order to support atomic writes
during file open (refer kiocb_set_rw_flags()). Set this capability on
a regular file if ext4 can do atomic write.

Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>