Zap cache_inode_weakref.

The use of weakrefs mad log-term reference simple, but came
added a couple of kinds of overhead.  Using ordinary FSAL
keys (augmented by exportid, see below) as dirent reference
removes a lookup table, and streamlines several code paths.

Thanks to Eldar Kleiner <eldark@infinidat.com> for the analysis.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>