fs: do not drop inode_lock in dispose_list

Despite the comment above it we can not safely drop the lock here.
invalidate_list is called from many other places that just umount.
Also switch to proper list macros now that we never drop the lock.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>