tune2fs: implement support for set/get label iocts

tune2fs: implement support for set/get label iocts

Implement support for FS_IOC_SETFSLABEL and FS_IOC_GETFSLABEL ioctls.
Try to use the ioctls if possible even before we open the file system
since we don't need it. Only fall back to the old method in the case the
file system is not mounted, is mounted read only in the set label case,
or the ioctls are not suppported by the kernel.

The new ioctls can also be supported by file system drivers other than
ext4. As a result tune2fs and e2label will work for those file systems
as well as long as the file system is mounted. Note that we still truncate
the label exceeds the supported lenghth on extN file system family, while
we keep the label intact for others.

Update tune2fs and e2label as well.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>