Patch from Andreas Gruenbacher <agruen@suse.de>

Tony Dziedzic has found two bugs in the extended attributes code. Patches with
explanations are attached.

1: Extended attribute sharing on ext2/ext3 not working

   The mb_cache_entry_insert function constantly returns an -EBUSY error
   instead of 0, which causes the xattr cache that is needed by the xattr
   sharing mechanism on ext2/ext3 to not share anything.  This patch fixes
   the problem.  (It is possible that after applying this fix we will hit
   bugs in code that wasn't used before.)

2: Oops in one of the xattr debug statements: The old_bh variable is NULL
   if an inode that previously had no EA's assigned would share an EA block
   with another inode.  (This was hidden by the xattr sharing bug).