ext4: fix potential unnitialized variable

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: 8016e29 ("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>

Author: Dan Carpenter

fs/ext4/mballoc.c

@@ -6126,6 +6126,7 @@ ext4_mb_new_blocks_simple(struct ext4_allocation_request *ar, int *errp)
 	ext4_mb_mark_bb(sb, block, 1, true);
 	ar->len = 1;
 
+	*errp = 0;
 	return block;
 }