btrfs: fix parameter documentation for btrfs_reserve_data_bytes()

btrfs: fix parameter documentation for btrfs_reserve_data_bytes()

We don't have a fs_info argument anymore since commit 5d39fda880be
("btrfs: pass btrfs_space_info to btrfs_reserve_data_bytes()"), it
was replaced by a space_info argument. So update the documentation.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <asj@kernel.org>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>