Btrfs: fix casting error in scrub reada code

The reada code from scrub was casting down a u64 to
an unsigned long so it could insert it into a radix tree.

What it really wanted to do was cast down the result of a shift, instead
of casting down the u64.  The bug resulted in trying to insert our
reada struct into the wrong place, which caused soft lockups and other
problems.

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