makefs: avoid warning when creating FAT filesystem on existing file

Previously the mkfs_msdos function (from newfs_msdos) emitted warnings
in the case that an image size is specified and the target is not a
file, or no size is specified and the target is not a character device.
The latter warning (not a character device) doesn't make sense when this
code is used in makefs, regardless of whether an image size is specified
or not.

Sponsored by:	The FreeBSD Foundation

sbin/newfs_msdos/mkfs_msdos.c

@@ -285,8 +285,10 @@ mkfs_msdos(const char *fname, const char *dtype, const struct msdos_options *op)
 	if (!S_ISREG(sb.st_mode))
 	    warnx("warning, %s is not a regular file", fname);
     } else {
+#ifndef MAKEFS
 	if (!S_ISCHR(sb.st_mode))
 	    warnx("warning, %s is not a character device", fname);
+#endif
     }
     if (!o.no_create)
 	if (check_mounted(fname, sb.st_mode) == -1)