fanotify: synchronize adding/removing a mark by means of group mutex

When removing flags from the mask of a mark the mask may go to 0 and the mark should
be destroyed. But right before we destroy it there is a race which allows a
concurrent process to add flags to the mask of the same marks that we are going to destroy.
Thus we might end up destroying a mark that has not all flags cleared in its mask.

This patch uses the group mutex to serialize the access to a mark, so that a concurrent
concurrent process can no longer access it if it is going to be destroyed.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: Eric Paris <eparis@redhat.com>