splice: Fix buffered splice of a blockdev

splice: Fix buffered splice of a blockdev

Fix the new filemap_splice_read() function to get i_size from
in->f_mapping->host, not in->f_inode so that it works with block devices
too (in->f_inode points to the device file, which is typically zero size).

Fixes: 07073eb01c5f ("splice: Add a func to do a splice from a buffered file without ITER_PIPE")
Link: https://lore.kernel.org/r/0c6b661c-f7ff-cf12-b7f0-00b6b2f1317b@amd.com/
Reported-by: Ayush Jain <ayush.jain3@amd.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Christoph Hellwig <hch@lst.de>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: David Hildenbrand <david@redhat.com>
cc: John Hubbard <jhubbard@nvidia.com>
cc: Steve French <stfrench@microsoft.com>
cc: linux-mm@kvack.org
cc: linux-block@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Tested-by: Ayush Jain <ayush.jain3@amd.com>
Link: https://lore.kernel.org/r/1770755.1681894451@warthog.procyon.org.uk
Signed-off-by: Jens Axboe <axboe@kernel.dk>