copy_file_range(2): Fix for small values of input file offset and len

r366302 broke copy_file_range(2) for small values of
input file offset and len.

It was possible for rem to be greater than len and then
"len - rem" was a large value, since both variables are
unsigned.

Approved by: re (gjb)

(cherry picked from commit a5f9fe2bab789f49e8b53da3a62dbd34725e23ea)