sunrpc: add rpc_lookup_generic_cred

Add function rpc_lookup_generic_cred, which allows lookups of a generic
credential that's not current_cred().

[jlayton: add gfp_t parm]

Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>