cache: drop the NULL check from VP2VNODELOCK

Now that negative entries are annotated with a dedicated flag, NULL vnodes
are no longer passed.

cache: depessimize hashing macros/inlines

All hash sizes are power-of-2, but the compiler does not know that for sure
and 'foo % size' forces doing a division.

Store the size - 1 and use 'foo & hash' instead which allows mere shift.

cache: move shrink lock init to nchinit

This gets rid of unnecesary sysinit usage.

While here also rename the lock to be consistent with the rest.

cache: sprinkle __predict_false