GFS2: Fix compiler warning from previous patch

This shouldn't really be required, but gcc can't tell that
"al" is only accessed when initialised.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
diff --git a/fs/gfs2/aops.c b/fs/gfs2/aops.c
index 1bf1788..6b24afb 100644
--- a/fs/gfs2/aops.c
+++ b/fs/gfs2/aops.c
@@ -615,7 +615,7 @@
 	unsigned int data_blocks = 0, ind_blocks = 0, rblocks;
 	int alloc_required;
 	int error = 0;
-	struct gfs2_alloc *al;
+	struct gfs2_alloc *al = NULL;
 	pgoff_t index = pos >> PAGE_CACHE_SHIFT;
 	unsigned from = pos & (PAGE_CACHE_SIZE - 1);
 	unsigned to = from + len;