nfsd: more robust allocation failure handling in nfsd_reply_cache_init

Currently, we try to allocate the cache as a single, large chunk, which
can fail if no big chunks of memory are available. We _do_ try to size
it according to the amount of memory in the box, but if the server is
started well after boot time, then the allocation can fail due to memory
fragmentation.

Fall back to doing a vzalloc if the kcalloc fails, and switch the
shutdown code to do a kvfree to handle freeing correctly.

Reported-by: Olaf Hering <olaf@aepfle.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>