xfs_destroy_workqueues() should not be tagged with__exit

ia64 throws away .exit sections for the built-in CONFIG case, so routines
that are used in other circumstances should not be tagged as __exit.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>