ext2fs: add support for e2di_uid_high and e2di_gid_high.

[pfg]

Summary

It is unclear to me exactly when they were introudced: it appears the are
part of ext3 but only officially available when a bigger inode is
supported ... basically when the extended timestamps are there.

[pfg]

Fix the shifts.
Move the extended inode changes to one unique if block.
Support both RW on ext3.

[pfg]

Update (again) the shifts: e2di_uid and e2di_gid are just uint16_t.

[pfg]

more bit mangling.

[fsu]

As, I learned linux sources you can use uid/gid high without E2DI_HAS_XTIME(ip) check, because it depends only from mount option on linux side, not from inode size.
So, seems like

ip->i_uid |= (uint32_t)ei->e2di_uid_high << 16;
ip->i_gid |= (uint32_t)ei->e2di_gid_high << 16;

could be moved out from if (E2DI_HAS_XTIME(ip)).

[pfg]

Applying feedback from Fedor.