fsnotify: add mutex destroy

Call mutex_destroy() before freeing notification group. This only adds
some additional debug checks when mutex debugging is enabled but still
it may be useful.

Link: https://lore.kernel.org/r/20200512181803.405832-1-fabf@skynet.be


Signed-off-by: Fabian Frederick <fabf@skynet.be>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>