xfs: move the legacy xfs_attr_list to xfs_ioctl.c

The old xfs_attr_list code is only used by the attrlist by handle
ioctl.  Move it to xfs_ioctl.c with its user.  Also move the
attrlist and attrlist_ent structure to xfs_fs.h, as they are exposed
user ABIs.  They are used through libattr headers with the same name
by at least xfsdump.  Also document this relation so that it doesn't
require a research project to figure out.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Chandan Rajendra <chandanrlinux@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>