fanotify: remove unneeded sub-zero check for unsigned value

Unsigned size_t len in copy_fid_info_to_user is checked
for negative value. This check is redundant as it is
always false.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 5e469c83 ("fanotify: copy event fid info to user")
Signed-off-by: Nikita Kiryushin <kiryushin@ancud.ru>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <d296ff1c-dcf7-4813-994b-3c4369debb7d@ancud.ru>