silence nfscache allocation warnings with kvzalloc

Currently nfsd_reply_cache_init attempts hash table allocation through
kmalloc, and manually falls back to vzalloc if that fails. This makes
the code a little larger than needed, and creates a significant amount
of serial console spam if you have enough systems.

Switching to kvzalloc gets rid of the allocation warnings, and makes
the code a little cleaner too as a side effect.

Freeing of nn->drc_hashtbl is already done using kvfree currently.
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>