xfs: fix potential infinite loop in xfs_iomap_prealloc_size()

If freesp == 0, we could end up in an infinite loop while squashing
the preallocation. Break the loop when we've killed the prealloc
entirely.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

(cherry picked from commit e78c420)

fs/xfs/xfs_iomap.c

@@ -413,7 +413,7 @@ xfs_iomap_prealloc_size(
 		 * have a large file on a small filesystem and the above
 		 * lowspace thresholds are smaller than MAXEXTLEN.
 		 */
-		while (alloc_blocks >= freesp)
+		while (alloc_blocks && alloc_blocks >= freesp)
 			alloc_blocks >>= 4;
 	}