[GFS2] Use atomic_t for journal free blocks counter

This patch changes the counter which keeps track of the free
blocks in the journal to an atomic_t in preparation for the
following patch which will update the log reservation code.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
diff --git a/fs/gfs2/incore.h b/fs/gfs2/incore.h
index 911822d1..7ae0206 100644
--- a/fs/gfs2/incore.h
+++ b/fs/gfs2/incore.h
@@ -595,7 +595,7 @@
 	struct list_head sd_log_le_databuf;
 	struct list_head sd_log_le_ordered;
 
-	unsigned int sd_log_blks_free;
+	atomic_t sd_log_blks_free;
 	struct mutex sd_log_reserve_mutex;
 
 	u64 sd_log_sequence;