Btrfs: Make the code for reading/writing free space cache generic

Extract out block group specific code from lookup_free_space_inode(),
create_free_space_inode(), load_free_space_cache() and
btrfs_write_out_cache(), so the code can be used to read/write
free ino cache.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>