UBI: fix a horrible memory deallocation bug

UBI was mistakingly using 'kfree()' instead of 'kmem_cache_free()' when
freeing "attach eraseblock" structures in vtbl.c. Thankfully, this happened
only when we were doing auto-format, so many systems were unaffected. However,
there are still many users affected.

It is strange, but the system did not crash and nothing bad happened when
the SLUB memory allocator was used. However, in case of SLOB we observed an
crash right away.

This problem was introduced in 2.6.39 by commit
"6c1e875c

 UBI: add slab cache for ubi_scan_leb objects"

A note for stable trees:
  Because variable were renamed, this won't cleanly apply to older kernels.
  Changing names like this should help:
	1. ai -> si
	2. aeb_slab_cache -> seb_slab_cache
	3. new_aeb -> new_seb

Reported-by: Richard Genoud <richard.genoud@gmail.com>
Tested-by: Richard Genoud <richard.genoud@gmail.com>
Tested-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: stable@vger.kernel.org [v2.6.39+]
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>