resize2fs: Do not fail if EXT4_IOC_RESIZE_FS ioctl doesn't exist

resize2fs: Do not fail if EXT4_IOC_RESIZE_FS ioctl doesn't exist

Commit 9f6ba888f027ba added support for new online resize ioctl
EXT4_IOC_RESIZE_FS. It is also trying to avoid failure when this
ioctl() is not supported by the kernel however it is checking wrong
error code (EINVAL).

When the ioctl does not exist, errno is set to ENOTTY, so we should
check for that, rather than EINVAL which means that ioctl arguments
are not valid. So change the code to check for ENOTTY and allow
resize2fs to try to use the old approach. Also add some comments.

Addresses-Red-Hat-Bugzilla: #746284
Addresses-Debian-Bug: #644989

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>