bcachefs: Fix -Wc23-extensions in bch2_check_dirents()

Clang warns (or errors with CONFIG_WERROR=y):

  fs/bcachefs/fsck.c:2325:2: error: label followed by a declaration is a C23 extension [-Werror,-Wc23-extensions]
   2325 |         int ret = bch2_trans_run(c,
        |         ^

On clang-17 and older, this is an unconditional error:

  fs/bcachefs/fsck.c:2325:2: error: expected expression
   2325 |         int ret = bch2_trans_run(c,
        |         ^

Move the declaration of ret to the top of the function to resolve both
ways this issue manifests.

Fixes: c72def52 ("bcachefs: Run check_dirents second time if required")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>