resize2fs: convert fs to and from 64bit mode

resize2fs: convert fs to and from 64bit mode

resize2fs does its magic by loading a filesystem, duplicating the
in-memory image of that fs, moving relevant blocks out of the way of
whatever new metadata get created, and finally writing everything back
out to disk.  Enabling 64bit mode enlarges the group descriptors,
which makes resize2fs a reasonable vehicle for taking care of the rest
of the bookkeeping requirements, so add to resize2fs the ability to
convert a filesystem to 64bit mode and back.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Cc: TR Reardon <thomas_reardon@hotmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>