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: d9b83c756953 ("libnvdimm, btt: rework error clearing")
Signed-off-by: Qian Cai <[e-mail]>
Reviewed-by: Vishal Verma <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dan Williams <[e-mail]>

drivers/nvdimm/btt.c

···
 
 		ret = btt_data_read(arena, page, off, postmap, cur_len);
 		if (ret) {
-			int rc;
-
 			/* Media error - set the e_flag */
-			rc = btt_map_write(arena, premap, postmap, 0, 1,
-				NVDIMM_IO_ATOMIC);
+			if (btt_map_write(arena, premap, postmap, 0, 1, NVDIMM_IO_ATOMIC))
+				dev_warn_ratelimited(to_dev(arena),
+					"Error persistently tracking bad blocks at %#x\n",
+					premap);
 			goto out_rtt;
 		}