btrfs: Streamline log_extent_csums a bit

Currently this function takes the root as an argument only to get the
log_root from it. Simplify this by directly passing the log root from
the caller. Also eliminate the fs_info local variable, since it's used
only once, so directly reference it from the transaction handle.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>