cygwin: pread/pwrite: prevent EBADF error after fork()

cygwin: pread/pwrite: prevent EBADF error after fork()

If the parent process has already used pread() or pwrite(), these
functions fail with EBADF if used on the inherited fd.  Ensure that
fix_after_fork() is called to invalidate the prw_handle.  This issue
has been detected by 'stress-ng --pseek 1'.

Fixes: c36cd56c548a ("* fhandler.cc (fhandler_base::open): Drop local create_options variable.")
Signed-off-by: Christian Franke <christian.franke@t-online.de>