xfs: combine xfs_dir_canenter into xfs_dir_createname

xfs_dir_canenter and xfs_dir_createname are
almost identical.

Fold the former into the latter, with a helpful
wrapper for the former.  If createname is called without
an inode number, it now only checks for space, and does
not actually add the entry.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>