MINOR: mworker: mworker_catch_sigchld: use fd_delete instead of close

MINOR: mworker: mworker_catch_sigchld: use fd_delete instead of close

If the worker exits due to failure or due to receiving TERM signal, in the
master context, we can't now simply close the master's fd (ipc_fd[0]) of
the inherited master CLI sockpair.

When the worker is created, in the master process context MASTER proxy listener
is bound to ipc_fd[0]. When this worker fails or exits, master process is
always in its polling loop. So, closing some fd in its context immediately
triggers the BUG_ON(fd->owner), as the poller try to reinsert the "freed" fd
into fdtab and try to reuse it. We must call fd_delete in this case. This will
deinitializes fd auxilary data and closes its properly.