rgw/posix: fix Inotify member initialization order race

rgw/posix: fix Inotify member initialization order race

wfd and efd were initialized in the Inotify constructor body, but
the inotify thread was started in the member initializer list via
thrd(&Inotify::ev_loop, this). Since C++ initializes members in
declaration order (wfd, efd, thrd), the thread could start before
the constructor body ran, causing ev_loop() to capture indeterminate
fd values into its local pollfd array.

When the destructor later signaled shutdown via the real efd, the
thread never woke because it was polling the wrong fd, causing
thrd.join() to block indefinitely. This predominantly affected arm64
builds due to the weaker memory model widening the race window.

Fix by moving wfd/efd initialization into the member initializer
list so they are set before the thread starts. Also make shutdown
std::atomic<bool> to eliminate the data race, close wfd/efd in the
destructor to fix the fd leak, and add error checking for eventfd().

Fixes: https://tracker.ceph.com/issues/75601
Signed-off-by: Kefu Chai <k.chai@proxmox.com>