Remove useless comparisions of assigned offset and resid with the
sources from uio.  Both uio_offset and offset, and uio_resid and resid
have the same types for some time.

Add check for buflen overflow by comparing the buflen with both offset
and resid (vs. comparing with offset only, as it is currently done).

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