extfs: remove redundant little endian conversion.

The XTIME_TO_NSEC macro already calls the htole32(), so there is no need
to call it twice. This code does nothing on LE platforms and affects only
nanosecond and birthtime fields so it's difficult to notice on regular use.

Hinted by: DragonFlyBSD (git ae503f8f6f4b9a413932ffd68be029f20c38cab4)

X-MFC with: r361136