btrfs: pass dentry to btrfs_mksubvol() and btrfs_mksnapshot()

There's no reason to pass 'struct path' to btrfs_mksubvol(), though it's
been like the since the first commit 76dda93c ("Btrfs: add
snapshot/subvolume destroy ioctl").  We only use the dentry so we should
pass it directly.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>