[BUG]
There is a compilation warning reported on commit ae76d8e3 ("btrfs:
scrub: fix grouping of read IO"), where gcc (14.0.0 20231022 experimental)
is reporting the following uninitialized variable:

  fs/btrfs/scrub.c: In function ‘scrub_simple_mirror.isra’:
  fs/btrfs/scrub.c:2075:29: error: ‘found_logical’ may be used uninitialized [-Werror=maybe-uninitialized[https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmaybe-uninitialized]]
   2075 |                 cur_logical = found_logical + BTRFS_STRIPE_LEN;
  fs/btrfs/scrub.c:2040:21: note: ‘found_logical’ was declared here
   2040 |                 u64 found_logical;
        |                     ^~~~~~~~~~~~~

[CAUSE]
This is a false alert, as @found_logical is passed as parameter
@found_logical_ret of function queue_scrub_stripe().

As long as queue_scrub_stripe() returned 0, we would update
@found_logical_ret.  And if queue_scrub_stripe() returned >0 or <0, the
caller would not utilized @found_logical, thus there should be nothing
wrong.

Although the triggering gcc is still experimental, it looks like the
extra check on "if (found_logical_ret)" can sometimes confuse the
compiler.

Meanwhile the only caller of queue_scrub_stripe() is always passing a
valid pointer, there is no need for such check at all.

[FIX]
Although the report itself is a false alert, we can still make it more
explicit by:

- Replace the check for @found_logical_ret with ASSERT()

- Initialize @found_logical to U64_MAX

- Add one extra ASSERT() to make sure @found_logical got updated

Link: https://lore.kernel.org/linux-btrfs/87fs1x1p93.fsf@gentoo.org/
Fixes: ae76d8e3 ("btrfs: scrub: fix grouping of read IO")
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>