bcache: Better alloc tracepoints

Change the invalidate tracepoint to indicate how much data we're invalidating,
and change the alloc tracepoints to indicate what offset they're for.

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

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

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

diff --cc include/trace/events/bcache.h
Simple merge