nfs: add error check in nfs_idmap_prepare_message()

Even though the caller of nfs_idmap_prepare_message() checks return
code in their side but it's better to add an error check for match_int()
so that we can avoid unnecessary operations when bad int arg is
detected.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>