Btrfs: remove unused wait queue in struct extent_buffer

The lock_wq wait queue is not used anywhere, therefore just remove it.
On a x86_64 system, this reduced sizeof(struct extent_buffer) from 320
bytes down to 296 bytes, which means a 4Kb page can now be used for
13 extent buffers instead of 12.

Signed-off-by: Filipe David Borba Manana <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/extent_io.h

···
 	 * to unlock
 	 */
 	wait_queue_head_t read_lock_wq;
-	wait_queue_head_t lock_wq;
 	struct page *pages[INLINE_EXTENT_BUFFER_PAGES];
 #ifdef CONFIG_BTRFS_DEBUG
 	struct list_head leak_list;