nbd: use list_for_each_entry_safe to make it more consolidated and readable

Thus the traverse of the loop may delete nodes, use the safe version.

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>