knfsd: fix broken length check in nfs4idmap.c

Obviously at some point we thought "error" represented the length when
positive.  This appears to be a long-standing typo.

Thanks to Prasad Potluri <pvp@us.ibm.com> for finding the problem and
proposing an earlier version of this patch.

Cc: Steve French <smfltc@us.ibm.com>
Cc: Prasad V Potluri <pvp@us.ibm.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>