freebsd/libshare: nfs: don't send SIGHUP to all processes

pidfile_open() sets *pidptr to -1 if the process currently holding
the lock is between pidfile_open() and pidfile_write(),
the subsequent kill(mountdpid) would potentially SIGHUP all
non-system processes except init: just sleep for half a millisecond
and try again in that case

Reviewed-by: Don Brady <don.brady@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12067