locks: typedef fl_owner_t to void *

fl_owner_t is a cookie that can store all kinds of different pointers,
so don't pretends it points to a file structure.

For now just change the typedef, but as a follow on this will allow
to get rids of lots of casts and eventually the typedef itself.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>