rhashtable: use likely for rhashtable lookup

rhashtable: use likely for rhashtable lookup

Sometimes, the result of the rhashtable_lookup() is expected to be found.
Therefore, we can use likely() for such cases.

Following new functions are introduced, which will use likely or unlikely
during the lookup:

 rhashtable_lookup_likely
 rhltable_lookup_likely

A micro-benchmark is made for these new functions: lookup a existed entry
repeatedly for 100000000 times, and rhashtable_lookup_likely() gets ~30%
speedup.

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