[XFS] allow enabling CONFIG_XFS_DEBUG

Back when I first submitted XFS for mainline inclusion we made the
decision that the debug code is far to extensive to be accidentally
enabled by users in mainline.  But then again it's often quite useful
to track problems down and hacking the makefile all the time is rather
annoying.  Given all the debug options with even more overhead like
lockdep or DEBUG_PAGE_ALLOC users (or rather developers) should know
by now what they're doing.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>