Modify the subvol and snapshot creation ioctls to work anywhere in the tree

Modify the subvol and snapshot creation ioctls to work anywhere in the tree

This changes the snapshot and subvol ioctl API and command lines so
that new snapshots and subvols can be created anywhere.

Subvolume creation hasn't changed much:

btrfsctl -S subvol_name directory

This creates a new subvolume under 'directory'

Snapshot creation looks the same, but is actually different:

btrfsctl -s full_path_to_new_snapshot file_or_dir

For example: btrfsctl -s /mnt/new_snap /mnt/subvol

Will create a new snapshot named new_snap under /mnt of the root
found in /mnt/subvol.  It always snapshots the entire root regardless of
which file or directory inside the root you give it.

Signed-off-by: Chris Mason <chris.mason@oracle.com>