[GFS2] Further updates to dir and logging code

This reduces the size of the directory code by about 3k and gets
readdir() to use the functions which were introduced in the previous
directory code update.

Two memory allocations are merged into one. Eliminates zeroing of some
buffers which were never used before they were initialised by
other data.

There is still scope for further improvement in the directory code.

On the logging side, a hand created mutex has been replaced by a
standard Linux mutex in the log allocation code.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/incore.h

@@ -634,8 +634,7 @@ struct gfs2_sbd {
 	struct list_head sd_log_le_databuf;
 
 	unsigned int sd_log_blks_free;
-	struct list_head sd_log_blks_list;
-	wait_queue_head_t sd_log_blks_wait;
+	struct mutex sd_log_reserve_mutex;
 
 	uint64_t sd_log_sequence;
 	unsigned int sd_log_head;