xfs: Fix wrong error codes being returned

xfs_{compat_,}attrmulti_by_handle could return an errno with incorrect
sign in some cases. While at it, make sure ENOMEM is returned instead of
E2BIG if kmalloc fails.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>