Allow it to take ERR_PTR() for server and return ERR_CAST() of it in
such case.  All callers used to open-code that...
Reviewed-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NAnna Schumaker <Anna.Schumaker@Netapp.com>