net: xfrm: fix shift-out-of-bounds in xfrm_get_default

Syzbot hit shift-out-of-bounds in xfrm_get_default. The problem was in
missing validation check for user data.

up->dirmask comes from user-space, so we need to check if this value
is less than XFRM_USERPOLICY_DIRMASK_MAX to avoid shift-out-of-bounds bugs.

Fixes: 2d151d39

 ("xfrm: Add possibility to set the default to block if we have no policy")
Reported-and-tested-by:  <syzbot+b2be9dd8ca6f6c73ee2d@syzkaller.appspotmail.com>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>