knfsd: simplify a 'while' condition in svcsock.c

This while loop has an overly complex condition, which performs a couple of
assignments.  This hurts readability.

We don't really need a loop at all.  We can just return -EAGAIN and (providing
we set SK_DATA), the function will be called again.

So discard the loop, make the complex conditional become a few clear function
calls, and hopefully improve readability.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>