resize2fs: allow meta_bg/64-bit file systems to be online resized

resize2fs: allow meta_bg/64-bit file systems to be online resized

Resize2fs can't handle resizing flex_bg file systems that do not have
the resize inode, but when the kernel adds support for resizing using
the meta_bg layout, we should allow it be able to resize the file
system.

So move the flex_bg/resize_inode check to the just before we start
doing the off-line resize, instead of doing it earlier where it would
prohibit these file systems for both on-line and off-line resizes.

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