fanotify: compare fsid when merging name event

When merging name events, fsids of the two involved events have to
match. Otherwise we could merge events from two different filesystems
and thus effectively loose the second event.

Backporting note: Although the commit cacfb956 introducing this bug
was merged for 5.7, the relevant code didn't get used in the end until
7e8283af ("fanotify: report parent fid + name + child fid") which
will be merged with this patch. So there's no need for backporting this.

Fixes: cacfb956 ("fanotify: record name info for FAN_DIR_MODIFY event")
Reported-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>