mkfs_ext2: reuse string

function                                             old     new   delta
mkfs_ext2_main                                      2376    2378      +2
.rodata                                           135872  135848     -24
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/1 up/down: 2/-24)             Total: -22 bytes

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
diff --git a/util-linux/mkfs_ext2.c b/util-linux/mkfs_ext2.c
index 388bd7b..1748a47 100644
--- a/util-linux/mkfs_ext2.c
+++ b/util-linux/mkfs_ext2.c
@@ -220,7 +220,7 @@
 	xmove_fd(xopen(argv[0], O_WRONLY), fd);
 	fstat(fd, &st);
 	if (!S_ISBLK(st.st_mode) && !(option_mask32 & OPT_F))
-		bb_error_msg_and_die("not a block device");
+		bb_error_msg_and_die("%s: not a block device", argv[0]);
 
 	// check if it is mounted
 	// N.B. what if we format a file? find_mount_point will return false negative since