mke2fs: disallow creating FS on a loop mounted file with no option

When /etc/mtab is a symlink of /proc/mounts, mke2fs without -FF option
can create a filesystem on the image file that is mounted.
According to mke2fs man page, we should specify -FF option in this case.

This patch protects filesystem from unintended mke2fs caused by human error.

How to reproduce:
  # mke2fs -t ext4 -Fq fs.img
  # mount -o loop fs.img /mnt/mp1
  # mke2fs -t ext4 -Fq fs.img && echo "mke2fs success"
  mke2fs success

Signed-off-by: Kazuya Mio <k-mio@sx.jp.nec.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/configure b/configure
index 738bd89..31ec8d3 100755
--- a/configure
+++ b/configure
@@ -10370,7 +10370,7 @@
 done
 
 fi
-for ac_header in  	dirent.h 	errno.h 	execinfo.h 	getopt.h 	malloc.h 	mntent.h 	paths.h 	semaphore.h 	setjmp.h 	signal.h 	stdarg.h 	stdint.h 	stdlib.h 	termios.h 	termio.h 	unistd.h 	utime.h 	linux/falloc.h 	linux/fd.h 	linux/major.h 	net/if_dl.h 	netinet/in.h 	sys/disklabel.h 	sys/file.h 	sys/ioctl.h 	sys/mkdev.h 	sys/mman.h 	sys/prctl.h 	sys/queue.h 	sys/resource.h 	sys/select.h 	sys/socket.h 	sys/sockio.h 	sys/stat.h 	sys/syscall.h 	sys/sysmacros.h 	sys/time.h 	sys/types.h 	sys/un.h 	sys/wait.h
+for ac_header in  	dirent.h 	errno.h 	execinfo.h 	getopt.h 	malloc.h 	mntent.h 	paths.h 	semaphore.h 	setjmp.h 	signal.h 	stdarg.h 	stdint.h 	stdlib.h 	termios.h 	termio.h 	unistd.h 	utime.h 	linux/falloc.h 	linux/fd.h 	linux/major.h 	linux/loop.h 	net/if_dl.h 	netinet/in.h 	sys/disklabel.h 	sys/file.h 	sys/ioctl.h 	sys/mkdev.h 	sys/mman.h 	sys/prctl.h 	sys/queue.h 	sys/resource.h 	sys/select.h 	sys/socket.h 	sys/sockio.h 	sys/stat.h 	sys/syscall.h 	sys/sysmacros.h 	sys/time.h 	sys/types.h 	sys/un.h 	sys/wait.h
 do :
   as_ac_Header=`$as_echo "ac_cv_header_$ac_header" | $as_tr_sh`
 ac_fn_c_check_header_mongrel "$LINENO" "$ac_header" "$as_ac_Header" "$ac_includes_default"