ext4: fix potential unnitialized variable

[ Upstream commit 3f4830abd236d0428e50451e1ecb62e14c365e9b ]

Smatch complains "err" can be uninitialized in the caller.

    fs/ext4/indirect.c:349 ext4_alloc_branch()
    error: uninitialized symbol 'err'.

Set the error to zero on the success path.

Fixes: 8016e29f43 ("ext4: fast commit recovery path")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/363a4673-0fb8-4adf-b4fb-90a499077276@moroto.mountain
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/ext4/mballoc.c

@@ -5684,6 +5684,7 @@ ext4_mb_new_blocks_simple(struct ext4_allocation_request *ar, int *errp)
 	ext4_mb_mark_bb(sb, block, 1, 1);
 	ar->len = 1;
 
+	*errp = 0;
 	return block;
 }