Redo r258088 to avoid relying on signed arithmetic overflow, since
compiler interprets this as an undefined behaviour.  Instead, ensure
that the sum of uio_offset and uio_resid is below OFF_MAX using the
operation which cannot overflow.

Reported and tested by:	pho
Discussed with:	bde
Approved by:	des (pseudofs maintainer)
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week