Btrfs: change btrfs_del_leaf to drop locks earlier

btrfs_del_leaf does two things.  First it removes the pointer in the
parent, and then it frees the block that has the leaf.  It has the
parent node locked for both operations.

But, it only needs the parent locked while it is deleting the pointer.
After that it can safely free the block without the parent locked.

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