xfs: fix a few compiler warnings with CONFIG_XFS_QUOTA=n

Andi Kleen reported that gcc-4.5 gives lots of warnings for him
inside the XFS code.  It turned out most of them are due to the
quota stubs beeing macros, and gcc now complaining about macros
evaluating to 0 that are not assigned to variables.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>