btrfs-util: when opening subvolume fds, always set O_NOFOLLOW

btrfs-util: when opening subvolume fds, always set O_NOFOLLOW

Some of the btrfs utility functions already used O_NOFOLLOW others
didn't. Let's streamline this, and refuse operation when we are called
for symlinks on "remove" and "snapshot" too.

In particular in the "remove" case following symlinks is a bad idea, and
is quite different from how unlink() and friends work, which always
remove the symlink, and not the destination, a logic we should follow
here too.