nfsd: correctly handle return value from nfsd_map_name_to_*

These functions return an nfs status, not a host_err.  So don't
try to convert  before returning.

This is a regression introduced by
3c726023; I fixed up two of the callers,
but missed these two.

Cc: stable@kernel.org
Reported-by: Herbert Poetzl <herbert@13thfloor.at>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>