fs: 9p: add generic splice_read file operations

The v9fs file operations were missing the splice_read operations, which
breaks sendfile() of files on such a filesystem. I discovered this while
trying to load an eBPF program using iproute2 inside a 'virtme' environment
which uses 9pfs for the virtual file system. iproute2 relies on sendfile()
with an AF_ALG socket to hash files, which was erroring out in the virtual
environment.

Since generic_file_splice_read() seems to just implement splice_read in
terms of the read_iter operation, I simply added the generic implementation
to the file operations, which fixed the error I was seeing. A quick grep
indicates that this is what most other file systems do as well.

Link: http://lkml.kernel.org/r/20201201135409.55510-1-toke@redhat.com
Fixes: 36e2c742 ("fs: don't allow splice read/write without explicit ops")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

fs/9p/vfs_file.c

@@ -655,6 +655,7 @@ const struct file_operations v9fs_cached_file_operations = {
 	.release = v9fs_dir_release,
 	.lock = v9fs_file_lock,
 	.mmap = v9fs_file_mmap,
+	.splice_read = generic_file_splice_read,
 	.fsync = v9fs_file_fsync,
 };
 
@@ -667,6 +668,7 @@ const struct file_operations v9fs_cached_file_operations_dotl = {
 	.lock = v9fs_file_lock_dotl,
 	.flock = v9fs_file_flock_dotl,
 	.mmap = v9fs_file_mmap,
+	.splice_read = generic_file_splice_read,
 	.fsync = v9fs_file_fsync_dotl,
 };
 
@@ -678,6 +680,7 @@ const struct file_operations v9fs_file_operations = {
 	.release = v9fs_dir_release,
 	.lock = v9fs_file_lock,
 	.mmap = generic_file_readonly_mmap,
+	.splice_read = generic_file_splice_read,
 	.fsync = v9fs_file_fsync,
 };
 
@@ -690,6 +693,7 @@ const struct file_operations v9fs_file_operations_dotl = {
 	.lock = v9fs_file_lock_dotl,
 	.flock = v9fs_file_flock_dotl,
 	.mmap = generic_file_readonly_mmap,
+	.splice_read = generic_file_splice_read,
 	.fsync = v9fs_file_fsync_dotl,
 };
 
@@ -701,6 +705,7 @@ const struct file_operations v9fs_mmap_file_operations = {
 	.release = v9fs_dir_release,
 	.lock = v9fs_file_lock,
 	.mmap = v9fs_mmap_file_mmap,
+	.splice_read = generic_file_splice_read,
 	.fsync = v9fs_file_fsync,
 };
 
@@ -713,5 +718,6 @@ const struct file_operations v9fs_mmap_file_operations_dotl = {
 	.lock = v9fs_file_lock_dotl,
 	.flock = v9fs_file_flock_dotl,
 	.mmap = v9fs_mmap_file_mmap,
+	.splice_read = generic_file_splice_read,
 	.fsync = v9fs_file_fsync_dotl,
 };