generic: Test page faults during read and write

generic: Test page faults during read and write

Some filesystems have problems when the buffer passed to read or
write is memory-mapped to the file being read from or written to,
and the buffer needs to be faulted in during the read or write.
That's not common, but filesystems are still required to cope with
it, and if they fail this test, then they will also fail more
complex scenarios that involve multiple files.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>