[PATCH v2 07/13] btrfs-progs: Fix Wmaybe-uninitialized warning

[Qu Wenruo <wqu@xxxxxxxx>]

GCC 8.2.1 will report the following warning with "make W=1":

  ctree.c: In function 'btrfs_next_sibling_tree_block':
  ctree.c:2990:21: warning: 'slot' may be used uninitialized in this function [-Wmaybe-uninitialized]
    path->slots[level] = slot;
    ~~~~~~~~~~~~~~~~~~~^~~~~~

The culprit is the following code:

	int slot;		<< Not initialized
	int level = path->lowest_level + 1;
 	BUG_ON(path->lowest_level + 1 >= BTRFS_MAX_LEVEL);
 	while(level < BTRFS_MAX_LEVEL) {
		slot = path->slots[level] + 1;
		^^^^^^ but we initialize @slot here.
		...
	}
	path->slots[level] = slot;

It's possible that compiler doesn't get enough hint for BUG_ON() on
lowest_level + 1 >= BTRFS_MAX_LEVEL case.

Fix it by using a do {} while() loop other than while() {} loop, to
ensure we will run the loop for at least once.

Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
---
 ctree.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/ctree.c b/ctree.c
index 46e2ccedc0bf..867e8b60b199 100644
--- a/ctree.c
+++ b/ctree.c
@@ -2966,7 +2966,7 @@ int btrfs_next_sibling_tree_block(struct btrfs_fs_info *fs_info,
 	struct extent_buffer *next = NULL;
 
 	BUG_ON(path->lowest_level + 1 >= BTRFS_MAX_LEVEL);
-	while(level < BTRFS_MAX_LEVEL) {
+	do {
 		if (!path->nodes[level])
 			return 1;
 
@@ -2986,7 +2986,7 @@ int btrfs_next_sibling_tree_block(struct btrfs_fs_info *fs_info,
 		if (!extent_buffer_uptodate(next))
 			return -EIO;
 		break;
-	}
+	} while (level < BTRFS_MAX_LEVEL);
 	path->slots[level] = slot;
 	while(1) {
 		level--;
-- 
2.19.2