net: optimize sblock(), sbunlock()

Instead of acquiring sockbuf::sb_mtx inside sblock() and sbunlock(), rely
on the caller to take the lock for us.  Expand existing lock hold regions
in callers to make it so.  This reduces acquisitions of sb_mtx.

As a side effect, copies to and from userspace are done under the lock.
This can affect MSG_NOWAIT with demand paging major faults, but these are
screwed anyway.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>