net: xfrm: fix shift-out-of-bounce

We need to check up->dirmask to avoid shift-out-of-bounce bug,
since up->dirmask comes from userspace.

Also, added XFRM_USERPOLICY_DIRMASK_MAX constant to uapi to inform
user-space that up->dirmask has maximum possible value

Fixes: 2d151d39 ("xfrm: Add possibility to set the default to block if we have no policy")
Reported-and-tested-by:  <syzbot+9cd5837a045bbee5b810@syzkaller.appspotmail.com>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>