fanotify: prefix should_merge()

Prefix function with fanotify_ like others.

Link: https://lore.kernel.org/r/20200512181715.405728-1-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>