autofs: use __kernel_write() for the autofs pipe writing

autofs got broken in some configurations by commit 13c164b1a186
("autofs: switch to kernel_write") because there is now an extra LSM
permission check done by security_file_permission() in rw_verify_area().

autofs is one if the few places that really does want the much more
limited __kernel_write(), because the write is an internal kernel one
that shouldn't do any user permission checks (it also doesn't need the
file_start_write/file_end_write logic, since it's just a pipe).

There are a couple of other cases like that - accounting, core dumping,
and splice - but autofs stands out because it can be built as a module.

As a result, we need to export this internal __kernel_write() function
again.

We really don't want any other module to use this, but we don't have a
"EXPORT_SYMBOL_FOR_AUTOFS_ONLY()". But we can mark it GPL-only to at
least approximate that "internal use only" for licensing.

While in this area, make autofs pass in NULL for the file position
pointer, since it's always a pipe, and we now use a NULL file pointer
for streaming file descriptors (see file_ppos() and commit 438ab720c675:
"vfs: pass ppos=NULL to .read()/.write() of FMODE_STREAM files")

This effectively reverts commits 9db977522449 ("fs: unexport
__kernel_write") and 13c164b1a186 ("autofs: switch to kernel_write").

Fixes: 13c164b1a186 ("autofs: switch to kernel_write")
Reported-by: Ondrej Mosnacek <[e-mail]>
Acked-by: Christoph Hellwig <[e-mail]>
Acked-by: Acked-by: Ian Kent <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/autofs/waitq.c

···
 
 	mutex_lock(&sbi->pipe_mutex);
 	while (bytes) {
-		wr = kernel_write(file, data, bytes, &file->f_pos);
+		wr = __kernel_write(file, data, bytes, NULL);
 		if (wr <= 0)
 			break;
 		data += wr;

fs/read_write.c

···
 	inc_syscw(current);
 	return ret;
 }
+/*
+ * This "EXPORT_SYMBOL_GPL()" is more of a "EXPORT_SYMBOL_DONTUSE()",
+ * but autofs is one of the few internal kernel users that actually
+ * wants this _and_ can be built as a module. So we need to export
+ * this symbol for autofs, even though it really isn't appropriate
+ * for any other kernel modules.
+ */
+EXPORT_SYMBOL_GPL(__kernel_write);
 
 ssize_t kernel_write(struct file *file, const void *buf, size_t count,
 			    loff_t *pos)