This bug was fixed in b487aca48535, but I did not have a regression
test ready for it at the time.
The test allocates a large buffer, faults part of it in and leaves the
rest unmapped, then tries to read a 3 GiB file into it. If the bug is
present, the read succeeds, but the return value exceeds the requested
amount by exactly one page less than the amount that was prefaulted.
PR: 298159
MFC after: 1 week