Cygwin: fork: fix a potential hang in fork

Cygwin: fork: fix a potential hang in fork

while debugging a problem introduced in commit
63b503916d42 ("Cygwin: tls_pathbuf: Use Windows heap")
a hang in fork was encountered using the original implementation
of tls_pathbuf:

Using tmp_pathbuf inside the code block guarded by __malloc_trylock
may call malloc from tmp_pathbuf::w_get and thus trying to lock an
exclusive SRW lock recursively, which results in a deadlock.

Allocate a small SECURITY_ATTRIBUTES block on the stack rather than
allocating a 64K tmp_pathbuf.  This avoids the potential malloc call.

Drop the __malloc_trylock call entirely.  There must not be a malloc
call inside the frok::parent block guarded by __malloc_lock, and
just trying to lock is too dangerous inside fork while other threads
might actually chage the content of the heap.  Additionally, add a
comment frowning on malloc usage inside tis code block.

Fixes: 44a79a6eca3d ("Cygwin: convert malloc lock to SRWLOCK")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>