fanotify: don't write with size under sizeof(response)

fanotify_write() only aligned copy_from_user size to sizeof(response)
for higher values. This patch avoids all values below as suggested
by Amir Goldstein and set to response size unconditionally.

Link: https://lore.kernel.org/r/20200512181921.405973-1-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>