erofs: support splice() in inode_share mode

erofs_ishare_fops routes everything that touches the page cache to the
backing file in ->private_data: read_iter clones the iocb onto it, mmap
does vma_set_file(), fadvise calls vfs_fadvise() on it.  splice_read was
left as filemap_splice_read(), which works on the user file's own mapping:
it does init_sync_kiocb(&iocb, in), and filemap_get_pages() then takes
iocb->ki_filp->f_mapping.

So splice() and sendfile() fill the per-inode page cache rather than the
shared one.  The content is the same either way, since erofs_fill_inode()
sets a_ops on that mapping too, which is why this went unnoticed.  Two
identical 8 MiB files under inode_share, reading one of them with splice(2)
alone, in pages:

  before   own 2048   shared 0
  after    own 0      shared 2048

"own" is cachestat(fd), which reports the file's own mapping; "shared" is
mmap()+mincore(), which erofs_ishare_mmap() redirects to the backing file.

Read through the backing file, as read_iter already does.

Link: https://lore.kernel.org/all/b7dc7192-d586-45a2-bc4a-b41dc681c9bb@linux.alibaba.com/
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <xiang@kernel.org>