xfs: validate xattr name earlier in recovery

When we're validating a recovered xattr log item during log recovery, we
should check the name before starting to allocate resources.  This isn't
strictly necessary on its own, but it means that we won't bother with
huge memory allocations during recovery if the attr name is garbage,
which will simplify the changes in the next patch.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>