bcache: Don't touch bucket gen for dirty ptrs

Unnecessary since a bucket that has dirty pointers pointing to it can
never be invalidated - and skipping it is a measurable performance
boost, since the bucket gen will usually be a cache miss.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

diff --cc drivers/md/bcache/bset.c
Simple merge

diff --cc drivers/md/bcache/request.c
Simple merge