knfsd: rename sk_defer_lock to sk_lock

Now that sk_defer_lock protects two different things, make the name more
generic.

Also don't bother with disabling _bh as the lock is only ever taken from
process context.

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>