shmem: Apply a couple of filemap_splice_read() fixes to shmem_splice_read()

Fix shmem_splice_read() to use the inode from in->f_mapping->host rather
than file_inode(in) and to skip the splice if it starts after s_maxbytes,
analogously with fixes to filemap_splice_read().

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

mm/shmem.c

diff --git a/mm/shmem.c b/mm/shmem.c
index 0164ccc..8a16d4c 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2780,13 +2780,16 @@ static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos,
 				      struct pipe_inode_info *pipe,
 				      size_t len, unsigned int flags)
 {
-	struct inode *inode = file_inode(in);
+	struct inode *inode = in->f_mapping->host;
 	struct address_space *mapping = inode->i_mapping;
 	struct folio *folio = NULL;
 	size_t total_spliced = 0, used, npages, n, part;
 	loff_t isize;
 	int error = 0;
 
+	if (unlikely(*ppos >= inode->i_sb->s_maxbytes))
+		return 0;
+
 	/* Work out how much data we can actually add into the pipe */
 	used = pipe_occupancy(pipe->head, pipe->tail);
 	npages = max_t(ssize_t, pipe->max_usage - used, 0);