ext2fs: Support e2di_uid_high and e2di_gid_high.

The fields exist on all versions of the filesystem and using them is a mount
option on linux. For FreeBSD, the corresponding i_uid and i_gid are always
long enough so use them by default.

Reviewed by: Fedor Uporov
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D11354