common/Preforker: fix broken recursion on exit(3)

If we exit via preforker, call exit(3) and not recursively back into
Preforker::exit(r).  Otherwise you get a hang with the child blocked
at:

Thread 1 (Thread 0x7fa08962e7c0 (LWP 5419)):
#0  0x000000309860e0cd in write () from /lib64/libpthread.so.0
#1  0x00000000005cc906 in Preforker::exit(int) ()
#2  0x00000000005c8dfb in main ()

and the parent at

#0  0x000000309860eba7 in waitpid () from /lib64/libpthread.so.0
#1  0x00000000005cc87a in Preforker::parent_wait() ()
#2  0x00000000005c75ae in main ()

Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>