bcache: Kill bucket->gc_gen

gc_gen was a temporary used to recalculate last_gc, but since we only need
bucket->last_gc when gc isn't running (gc_mark_valid = 1), we can just update
last_gc directly.

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

diff --cc drivers/md/bcache/bcache.h
Simple merge

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

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

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