Btrfs: insert_new_root: Fix lock type of the extent buffer.

btrfs_alloc_tree_block() returns an extent buffer on which a blocked lock has
been taken. Hence assign the appropriate value to path->locks[level].

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Signed-off-by: Chris Mason <clm@fb.com>