In r243868, the error message buffer errmsg have been changed from

an on-stack array to a pointer and therefore sizeof(errmsg) would
become 4 or 8 bytes depending on the architecture.

Fix this by using ERRMSGL in place of sizeof().

Submitted by:	J David <j.david.lists@gmail.com>
MFC after:	3 days
Approved by:	re (kib)

sys/kern/vfs_mountroot.c

@@ -710,7 +710,7 @@ parse_mount(char **conf)
 	errmsg = malloc(ERRMSGL, M_TEMP, M_WAITOK | M_ZERO);
 
 	if (vfs_byname(fs) == NULL) {
-		strlcpy(errmsg, "unknown file system", sizeof(errmsg));
+		strlcpy(errmsg, "unknown file system", ERRMSGL);
 		error = ENOENT;
 		goto out;
 	}