Squashfs: Use vmalloc rather than kmalloc for zlib workspace

Bugzilla bug 31422 reports occasional "page allocation failure. order:4"
at Squashfs mount time.  Fix this by making zlib workspace allocation
use vmalloc rather than kmalloc.

Reported-by: Mehmet Giritli <mehmet@giritli.eu>
Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>