xfs: remove incorrect error negation in attr_multi ioctl

xfs_compat_attrmulti_by_handle() calls memdup_user() which returns a
negative error code. The error code is negated by the caller and thus
incorrectly converted to a positive error code.

Remove the error negation such that the negative error is passed
correctly back up to userspace.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>