linux-user: Hold the fd-trans lock across fork

linux-user: Hold the fd-trans lock across fork

If another thread is holding target_fd_trans_lock during a fork,
then the lock becomes permanently locked in the child and the
emulator deadlocks at the next interaction with the fd-trans table.
As with other locks, acquire the lock in fork_start() and release
it in fork_end().

Cc: qemu-stable@nongnu.org
Signed-off-by: Geoffrey Thomas <geofft@ldpreload.com>
Fixes: c093364f4d91 "fd-trans: Fix race condition on reallocation of the translation table."
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2846
Buglink: https://github.com/astral-sh/uv/issues/6105
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20250314124742.4965-1-geofft@ldpreload.com>
(cherry picked from commit e4e839b2eeea5745c48ce47144c7842eb7cd455f)
(Mjt: adjust context in linux/main.c for 7.2)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>