nfsd_splice_actor(): handle compound pages

pipe_buffer might refer to a compound page (and contain more than a PAGE_SIZE
worth of data).  Theoretically it had been possible since way back, but
nfsd_splice_actor() hadn't run into that until copy_page_to_iter() change.
Fortunately, the only thing that changes for compound pages is that we
need to stuff each relevant subpage in and convert the offset into offset
in the first subpage.

Acked-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: Benjamin Coddington <bcodding@redhat.com>
Fixes: f0f6b614

 "copy_page_to_iter(): don't split high-order page in case of ITER_PIPE"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>