block/nfs: fix url parameter checking

this patch fixes the incorrect usage of strncmp and
adds simple error checking by means of parse_uint_full
instead of atoi for the supplied URL parameters.

Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
