NFSD: Fix reads with a non-zero offset that don't end on a page boundary

This was found when virtual machines with nfs-mounted qcow2 disks
failed to boot properly.

Reported-by: Anders Blomdell <anders.blomdell@control.lth.se>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2142132


Fixes: bfbfb618 ("nfsd_splice_actor(): handle compound pages")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>