btrfs: preparation to fixing mount/umount race

We need fs_info and root to live until the moment when the victim
superblock leaves the list, so we need to postpone free_fs_info()
until after ->put_super().  The call is buried in close_ctree(),
though, so we need to lift it into the callers (including
btrfs_put_super()) first.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>