Btrfs: move d_instantiate outside the transaction during mksubvol

Dave Sterba triggered a lockdep complaint about lock ordering
between the sb_internal lock and the cleaner semaphore.

btrfs_lookup_dentry() checks for orphans if we're looking up
the inode for a subvolume, and subvolume creation is triggering
the lookup with a transaction running.

This commit moves the d_instantiate after the transaction closes.

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