Hold a read lock on the map while copying out data during a sysctl(2) call

Hold a read lock on the map while copying out data during a sysctl(2) call
to prevent another thread from unmapping the memory and triggering
an assertion or even corrupting random physical memory pages.

This fix is similar to the change in uvm_glue.c rev. 1.74.  However in this
case we need to be careful since some sysctl(2) calls look at the map of
the current process.  In those cases we must not attempt to lock the map
again.

ok mpi@

Should fix:
Reported-by: syzbot+be89fe83d6c004fcb412@syzkaller.appspotmail.com