xfs: switch xfs_attrmulti_attr_get to lazy attr buffer allocation

Let the low-level attr code only allocate the needed buffer size
for xfs_attrmulti_attr_get instead of allocating the upper bound
at the top of the call chain.

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