[patch 1/3] FS_MBCACHE: don't needlessly make it built-in

Assume you have:
- one or more of ext2/3/4 statically built into your kernel
- none of these with extended attributes enabled and
- want to add onother one of ext2/3/4 modular and with
  extended attributes enabled

then you currently have to reboot to use it since this results in
CONFIG_FS_MBCACHE=y.

That's not a common issue, but I just ran into it and since there's no
reason to get a built-in mbcache in this case this patch fixes it.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Cc: Andreas Gruenbacher <agruen@suse.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>