9p: fix mmap to be read-only

v9fs was allowing writable mmap which could lead to kernel BUG() cases.
This sets the mmap function to generic_file_readonly_mmap which (correctly)
returns an error to applications which open mmap for writing.
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

fs/9p/vfs_file.c

@@ -184,7 +184,7 @@ static const struct file_operations v9fs_cached_file_operations = {
 	.open = v9fs_file_open,
 	.release = v9fs_dir_release,
 	.lock = v9fs_file_lock,
-	.mmap = generic_file_mmap,
+	.mmap = generic_file_readonly_mmap,
 };
 
 const struct file_operations v9fs_file_operations = {
@@ -194,5 +194,5 @@ const struct file_operations v9fs_file_operations = {
 	.open = v9fs_file_open,
 	.release = v9fs_dir_release,
 	.lock = v9fs_file_lock,
-	.mmap = generic_file_mmap,
+	.mmap = generic_file_readonly_mmap,
 };