bcache: Break up struct search

With all the recent refactoring around struct btree op struct search has
gotten rather large.

But we can now easily break it up in a different way - we break out
struct btree_insert_op which is for inserting data into the cache, and
that's now what the copying gc code uses - struct search is now specific
to request.c

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

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

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

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

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

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

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

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

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

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