btrfs: improve parameter description for __btrfs_write_out_cache

Fixes following W=1 warnings:
fs/btrfs/free-space-cache.c:1317: warning: Function parameter or member 'root' not described in '__btrfs_write_out_cache'
fs/btrfs/free-space-cache.c:1317: warning: Function parameter or member 'inode' not described in '__btrfs_write_out_cache'
fs/btrfs/free-space-cache.c:1317: warning: Function parameter or member 'ctl' not described in '__btrfs_write_out_cache'
fs/btrfs/free-space-cache.c:1317: warning: Function parameter or member 'block_group' not described in '__btrfs_write_out_cache'
fs/btrfs/free-space-cache.c:1317: warning: Function parameter or member 'io_ctl' not described in '__btrfs_write_out_cache'
fs/btrfs/free-space-cache.c:1317: warning: Function parameter or member 'trans' not described in '__btrfs_write_out_cache'

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