Remove some harmless cruft from vn_generic_copy_file_range().

An earlier version of the patch had code that set "error" between
line#s 2797-2799. When that code was moved, the second check for "error != 0"
could never be true and the check became harmless cruft.
This patch removes the cruft, mainly to make Coverity happy.

Reported by: asomers, cem