xen-netback: fix debugfs write length check

Enlarge buffer size and check input length properly, so that we don't
misuse -ENOSPC.

Note that command like "kickXXXX" is still allowed, that's one patch for
another day if we really want to be very strict on this.

Reported-by: SeeChen Ng <seechen81@gmail.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Zoltan Kiss <zoltan.kiss@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>