staging: p9auth: prevent some oopses and memory leaks

Before all testcases, do:
	mknod /dev/caphash c 253 0
	mknod /dev/capuse c 253 1

This patch does the following:

1. caphash write of > CAP_NODE_SIZE bytes overruns node_ptr->data
	(test: cat /etc/mime.types > /dev/caphash)
2. make sure we don't dereference a NULL cap_devices[0].head
	(test: cat serge@root@abab > /dev/capuse)
3. don't let strlen dereference a NULL target_user etc
	(test: echo ab > /dev/capuse)
4. Don't leak a bunch of memory in cap_write().  Note that
   technically node_ptr is not needed for the capuse write case.
   As a result I have a much more extensive patch splitting up
   cap_write(), but I thought a smaller patch that is easier to test
   and verify would be a better start.  To test:
	cnt=0
	while [ 1 ]; do
		echo /etc/mime.types > /dev/capuse
		if [ $((cnt%25)) -eq 0 ]; then
			head -2 /proc/meminfo
		fi
		cnt=$((cnt+1))
		sleep 0.3
	done
   Without this patch, it MemFree steadily drops.  With the patch,
   it does not.

I have *not* tested this driver (with or without these patches)
with factotum or anything - only using the tests described above.

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>