Cygwin: pipe: Fix a regression that raw_write() slows down

After the commit 7f3c22532577, writing to pipe extremely slows down.
This is because cygwait(select_sem, 10, cw_cancel) is called even
when write operation is already completed. With this patch, the
cygwait() is called only if the write operation is not completed.

Backported-from: 37ab3e0d55 (Cygwin: pipe: Fix a regression that raw_write() slows down, 2024-09-01)
Addresses: https://cygwin.com/pipermail/cygwin/2024-August/256398.html
Fixes: 7f3c22532577 ("Cygwin: pipe: handle signals explicitely in raw_write")
Reported-by: Jim Reisert AD1C <jjreisert@alum.mit.edu>
Reviewed-by: Corinna Vinschen <corinna@vinschen.de>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
(cherry picked from commit f78009cb1ccf84cc343cf2441c76196461d87532)
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/release/3.5.5

@@ -4,3 +4,6 @@ Fixes:
 - Fix undesired behaviour of console master thread in win32-input-mode
   which is supported by Windows Termainal.
   Addresses: https://cygwin.com/pipermail/cygwin/2024-August/256380.html
+
+- Fix a regression in 3.5.4 that writing to pipe extremely slows down.
+  Addresses: https://cygwin.com/pipermail/cygwin/2024-August/256398.html