qcow2: Assert that refcount block offsets fit in the refcount table

qcow2: Assert that refcount block offsets fit in the refcount table

Refcount table entries have a field to store the offset of the
refcount block. The rest of the bits of the entry are currently
reserved.

The offset is always taken from the entry using REFT_OFFSET_MASK to
ensure that we only use the bits that belong to that field.

While that mask is used every time we read from the refcount table, it
is never used when we write to it. Due to the other constraints of the
qcow2 format QEMU can never produce refcount block offsets that don't
fit in that field so any such offset when allocating a refcount block
would indicate a bug in QEMU.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>