btrfs: initialize start_slot in btrfs_log_prealloc_extents

btrfs: initialize start_slot in btrfs_log_prealloc_extents

Jens reported a compiler warning when using
CONFIG_CC_OPTIMIZE_FOR_SIZE=y that looks like this

  fs/btrfs/tree-log.c: In function ‘btrfs_log_prealloc_extents’:
  fs/btrfs/tree-log.c:4828:23: warning: ‘start_slot’ may be used
  uninitialized [-Wmaybe-uninitialized]
   4828 |                 ret = copy_items(trans, inode, dst_path, path,
	|                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   4829 |                                  start_slot, ins_nr, 1, 0);
	|                                  ~~~~~~~~~~~~~~~~~~~~~~~~~
  fs/btrfs/tree-log.c:4725:13: note: ‘start_slot’ was declared here
   4725 |         int start_slot;
	|             ^~~~~~~~~~

The compiler is incorrect, as we only use this code when ins_len > 0,
and when ins_len > 0 we have start_slot properly initialized.  However
we generally find the -Wmaybe-uninitialized warnings valuable, so
initialize start_slot to get rid of the warning.

Reported-by: Jens Axboe <axboe@kernel.dk>
Tested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>