libnvdimm/btt: fix variable 'rc' set but not used

drivers/nvdimm/btt.c: In function 'btt_read_pg':
drivers/nvdimm/btt.c:1264:8: warning: variable 'rc' set but not used
[-Wunused-but-set-variable]
    int rc;
        ^~

Add a ratelimited message in case a storm of errors is encountered.

Fixes: d9b83c75 ("libnvdimm, btt: rework error clearing")
Signed-off-by: Qian Cai <cai@lca.pw>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Link: https://lore.kernel.org/r/1572530719-32161-1-git-send-email-cai@lca.pw


Signed-off-by: Dan Williams <dan.j.williams@intel.com>