nbd: change a parameter's type to remove a memcpy call

This memcpy looks so strange, in fact it's merely a pointer dereference, so I
change the parameter's type to refer it more directly, this could make the
memcpy not needed anymore.

In the function nbd_read_stat where nbd_find_request is only once called, the
parameter served should be transformed accordingly.

Signed-off-by: Denis Cheng <crquan@gmail.com>
Cc: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>