BTREE_TRIGGER_WANTS_OLD_AND_NEW didn't work correctly when the old and
new key were both alloc keys, but different versions - it required old
and new key type to be identical, and this bug is a problem for the new
allocator rewrite.

This patch fixes it by checking if the old and new key have the same
trigger functions - the different versions of alloc (and inode) keys
have the same trigger functions.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>