drivers/md/bcache/super.c: use kvmalloc

drivers/md/bcache/super.c: use kvmalloc

bcache_device_init uses kmalloc for small requests and vmalloc for those
which are larger than 64 pages.  This alone is a strange criterion.
Moreover kmalloc can fallback to vmalloc on the failure.  Let's simply
use kvmalloc instead as it knows how to handle the fallback properly

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Michal Hocko <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Kent Overstreet <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>