bcache: safeguard a dangerous addressing in closure_queue

The use of the union reduces the size of closure struct by taking advantage
of the current size of its members. The offset of func in work_struct
equals the size of the first three members, so that work.work_func will
just reference the forth member - fn.

This is smart but dangerous. It can be broken if work_struct or the other
structs get changed, and can be a bit difficult to debug.

Signed-off-by: Liang Chen <liangchen.linux@gmail.com>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>