tune2fs: Fix "tune2fs -j <dev>" for extent-enabled filesystems

tune2fs: Fix "tune2fs -j <dev>" for extent-enabled filesystems

For filesystms that have the extent feature enabled, we need to grab
the use EXT2_IOC_GETFLAGS so that we don't accidentally end up trying
to request clearing the EXT2_EXTENT_FL, which is not supported and
causes the tune2fs -j error out.

Also fix the error returning in ext2fs_add_journal_inode() so it
returns a proper error code if the fstat() or ioctl() calls fail.

Addresses-Launchpad-bug: #416648

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