vfs_btrfs: add snapshot create/delete calls

vfs_btrfs: add snapshot create/delete calls

The "btrfs: manipulate snapshots" smb.conf parameter is disabled by
default, to encourage use of, and pass requests through to, the
vfs_snapper module.
When enabled, issue BTRFS_IOC_SNAP_CREATE_V2 and BTRFS_IOC_SNAP_DESTROY
ioctls accordingly. The ioctls are issued as root, so rely on permission
checks in the calling FSRVP server process.

Base share paths must exist as btrfs subvolumes in order to
be supported for snapshot operations.

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>