xfs: move xfs_attr_use_log_assist out of xfs_log.c

xfs: move xfs_attr_use_log_assist out of xfs_log.c

Source kernel commit: d9c61ccb3b09d8f892cccbf662ce0c870f8e4ade

The LARP patchset added an awkward coupling point between libxfs and
what would be libxlog, if the XFS log were actually its own library.
Move the code that enables logged xattr updates out of "lib"xlog and into
xfs_xattr.c so that it no longer has to know about xlog_* functions.

While we're at it, give xfs_xattr.c its own header file.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>