fsnotify: Avoid data race between fsnotify_recalc_mask() and fsnotify_object_watched()

When __fsnotify_recalc_mask() recomputes the mask on the watched object,
the compiler can "optimize" the code to perform partial updates to the
mask (including zeroing it at the beginning). Thus places checking
the object mask without conn->lock such as fsnotify_object_watched()
could see invalid states of the mask. Make sure the mask update is
performed by one memory store using WRITE_ONCE().

Reported-by:  <syzbot+701037856c25b143f1ad@syzkaller.appspotmail.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://lore.kernel.org/all/CACT4Y+Zk0ohwwwHSD63U2-PQ=UuamXczr1mKBD6xtj2dyYKBvA@mail.gmail.com


Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://patch.msgid.link/20240717140623.27768-1-jack@suse.cz