keep kernel module symbol table

vim drivers/md/bcache/Makefile:

obj-$(CONFIG_BCACHE)    += escache.o

ccflags-y := -O1

escache-y       := alloc.o bset.o btree.o closure.o debug.o extents.o\
    io.o journal.o movinggc.o request.o stats.o super.o sysfs.o trace.o\
    util.o writeback.o

CFLAGS_request.o    += -Iblock