ext4: Use slab allocator for sub-page sized allocations

Now that the SLUB seems to be fixed so that it respects the requested
alignment, use kmem_cache_alloc() to allocator if the block size of
the buffer heads to be allocated is less than the page size.
Previously, we were using 16k page on a Power system for each buffer,
even when the file system was using 1k or 4k block size.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>