shmem: Fix splice of a missing page

Fix shmem_splice_read() to splice only part of the partial page at the end
of a splice and not all of it.

This can be seen by splicing from a tmpfs file that's not a multiple of
PAGE_SIZE in size.  Without this fix, it splices extra data to round up to
PAGE_SIZE.

Fixes: bd194b187115 ("shmem: Implement splice-read")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Hugh Dickins <hughd@google.com>
cc: Christoph Hellwig <hch@lst.de>
cc: Jens Axboe <axboe@kernel.dk>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: John Hubbard <jhubbard@nvidia.com>
cc: David Hildenbrand <david@redhat.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: Chuck Lever <chuck.lever@oracle.com>
cc: linux-block@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org