xfs: Fix error path in xfs_get_acl

Error codes from xfs_attr_get other than -ENOATTR were not properly
reported.  Fix that.

In addition, the declaration of struct xfs_inode in xfs_acl.h isn't needed.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>