GFS2: Fall back to vmalloc if kmalloc fails for dir hash tables

This version has one more correction: the vmalloc calls are replaced
by __vmalloc calls to preserve the GFP_NOFS flag.

When GFS2's directory management code allocates buffers for a
directory hash table, if it can't get the memory it needs, it
currently gives a bad return code. Rather than giving an error,
this patch allows it to use virtual memory rather than kernel
memory for the hash table. This should make it possible for
directories to function properly, even when kernel memory becomes
very fragmented.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>