lib/rhashtable: guarantee initial hashtable allocation

rhashtable_init() may fail due to -ENOMEM, thus making the entire api
unusable.  This patch removes this scenario, however unlikely.  In order
to guarantee memory allocation, this patch always ends up doing
GFP_KERNEL|__GFP_NOFAIL for both the tbl as well as
alloc_bucket_spinlocks().

Upon the first table allocation failure, we shrink the size to the
smallest value that makes sense and retry with __GFP_NOFAIL semantics.
With the defaults, this means that from 64 buckets, we retry with only 4.
Any later issues regarding performance due to collisions or larger table
resizing (when more memory becomes available) is the least of our
problems.

Link: http://lkml.kernel.org/r/20180712185241.4017-9-manfred@colorfullife.com
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>