btrfs: refactor unlock_up

The purpose of this function is to unlock all nodes in a btrfs path
which are above 'lowest_unlock' and whose slot used is different than 0.
As such it used slightly awkward structure of 'if' as well as somewhat
cryptic "no_skip" control variable which denotes whether we should
check the current level of skipability or no.

This patch does the following (cosmetic) refactorings:

* Renames 'no_skip' to 'check_skip' and makes it a boolean. This
  variable controls whether we are below the lowest_unlock/skip_level
  levels.

* Consolidates the 2 conditions which warrant checking whether the
  current level should be skipped under 1 common if (check_skip) branch,
  this increase indentation level but is not critical.

* Consolidates the 'skip_level < i && i >= lowest_unlock' and
  'i >= lowest_unlock && i > skip_level' condition into a common branch
  since those are identical.

* Eliminates the local extent_buffer variable as in this case it doesn't
  bring anything to function readability.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -1349,33 +1349,34 @@ static noinline void unlock_up(struct btrfs_path *path, int level,
 {
 	int i;
 	int skip_level = level;
-	int no_skips = 0;
-	struct extent_buffer *t;
+	bool check_skip = true;
 
 	for (i = level; i < BTRFS_MAX_LEVEL; i++) {
 		if (!path->nodes[i])
 			break;
 		if (!path->locks[i])
 			break;
-		if (!no_skips && path->slots[i] == 0) {
+
+		if (check_skip) {
+			if (path->slots[i] == 0) {
 				skip_level = i + 1;
 				continue;
 			}
-		if (!no_skips && path->keep_locks) {
+
+			if (path->keep_locks) {
 				u32 nritems;
-			t = path->nodes[i];
-			nritems = btrfs_header_nritems(t);
+
+				nritems = btrfs_header_nritems(path->nodes[i]);
 				if (nritems < 1 || path->slots[i] >= nritems - 1) {
 					skip_level = i + 1;
 					continue;
 				}
 			}
-		if (skip_level < i && i >= lowest_unlock)
-			no_skips = 1;
+		}
 
-		t = path->nodes[i];
 		if (i >= lowest_unlock && i > skip_level) {
-			btrfs_tree_unlock_rw(t, path->locks[i]);
+			check_skip = false;
+			btrfs_tree_unlock_rw(path->nodes[i], path->locks[i]);
 			path->locks[i] = 0;
 			if (write_lock_level &&
 			    i > min_write_lock_level &&