rhashtable: Use __always_inline instead of inline

Sometimes, the compiler is not clever enough to inline the
rhashtable_lookup() for us, even if the "obj_cmpfn" and "key_len" in
params is const. This can introduce more overhead.

Therefore, use __always_inline for the rhashtable.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>