Staging: sep: potential buffer overflow in ioctl

tail_size is determined by several variables that come from the user
so we should verify that it's not too large.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>