mke2fs: don't fail creating the journal if /etc/mtab is missing

mke2fs: don't fail creating the journal if /etc/mtab is missing

The ext2fs_add_journal_inode() function calls
ext2fs_check_mount_point(), which can fail if /etc/mtab is missing.
This causes mke2fs to fail in the middle of the file system format
process; mke2fs calls ext2fs_check_mount_point() already (and has
appropriate fallbacks that calls fails), so add a flag so that mke2fs
can request ext2fs_add_journal_inode() to skip trying to call
e2fsck_check_mount_point().

Addresses-Sourceforge-Bug: #3509398

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>