inotify: fix race when adding a new watch

inotify: fix race when adding a new watch

In inotify_new_watch() the number of watches for a group is compared
against the max number of allowed watches and increased afterwards.  The
check and incrementation is not done atomically, so it is possible for
multiple concurrent threads to pass the check and increment the number of
marks above the allowed max.

This patch uses an inotify groups mark_lock to ensure that both check and
incrementation are done atomic.  Furthermore we dont have to worry about
the race that allows a concurrent thread to add a watch just after
inotify_update_existing_watch() returned with -ENOENT anymore, since this
is also synchronized by the groups mark mutex now.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Cc: Eric Paris <eparis@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>