sysctl: pass kernel pointers to ->proc_handler

Instead of having all the sysctl handlers deal with user pointers, which
is rather hairy in terms of the BPF interaction, copy the input to and
from  userspace in common code.  This also means that the strings are
always NUL-terminated by the common code, making the API a little bit
safer.

As most handler just pass through the data to one of the common handlers
a lot of the changes are mechnical.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/kernel/umh.c b/kernel/umh.c
index 7f255b5..9788ed4 100644
--- a/kernel/umh.c
+++ b/kernel/umh.c
@@ -630,7 +630,7 @@ int call_usermodehelper(const char *path, char **argv, char **envp, int wait)
 EXPORT_SYMBOL(call_usermodehelper);
 
 static int proc_cap_handler(struct ctl_table *table, int write,
-			 void __user *buffer, size_t *lenp, loff_t *ppos)
+			 void *buffer, size_t *lenp, loff_t *ppos)
 {
 	struct ctl_table t;
 	unsigned long cap_array[_KERNEL_CAPABILITY_U32S];