mtd: phram: Prevent divide by zero bug in phram_setup()

The problem is that "erasesize" is a uint64_t type so it might be
non-zero but the lower 32 bits are zero so when it's truncated,
"(uint32_t)erasesize", then that value is zero. This leads to a
divide by zero bug.

Avoid the bug by delaying the divide until after we have validated
that "erasesize" is non-zero and within the uint32_t range.

Fixes: dc2b3e5 ("mtd: phram: use div_u64_rem to stop overwrite len in phram_setup")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220121115505.GI1978@kadam

Author: Dan Carpenter

drivers/mtd/devices/phram.c

@@ -264,16 +264,20 @@ static int phram_setup(const char *val)
 		}
 	}
 
-	if (erasesize)
-		div_u64_rem(len, (uint32_t)erasesize, &rem);
-
 	if (len == 0 || erasesize == 0 || erasesize > len
-	    || erasesize > UINT_MAX || rem) {
+	    || erasesize > UINT_MAX) {
 		parse_err("illegal erasesize or len\n");
 		ret = -EINVAL;
 		goto error;
 	}
 
+	div_u64_rem(len, (uint32_t)erasesize, &rem);
+	if (rem) {
+		parse_err("len is not multiple of erasesize\n");
+		ret = -EINVAL;
+		goto error;
+	}
+
 	ret = register_device(name, start, len, (uint32_t)erasesize);
 	if (ret)
 		goto error;