btrfs: remove bogus NULL checks in __btrfs_write_out_cache()

btrfs: remove bogus NULL checks in __btrfs_write_out_cache()

Dan reported a new smatch warning in free-space-cache.c:

New smatch warnings:
fs/btrfs/free-space-cache.c:1207 write_pinned_extent_entries() warn: variable dereferenced before check 'block_group' (see line 1203)

But the check if the block_group pointer is NULL is bogus, because to
get to this point block_group::io_ctl has already been dereferenced
further up the call-chain when calling __btrfs_write_out_cache() from
btrfs_write_out_cache().

Remove the bogus checks for block_group == NULL in
__btrfs_write_out_cache() and it's siblings.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202601170636.WsePMV5H-lkp@intel.com/
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>