x86/mm: Add vsyscall address helper

We will shortly be using this check in two locations.  Put it in
a helper before we do so.

Let's also insert PAGE_MASK instead of the open-coded ~0xfff.
It is easier to read and also more obviously correct considering
the implicit type conversion that has to happen when it is not
an implicit 'unsigned long'.

Cc: [email protected]
Cc: Jann Horn <[email protected]>
Cc: Sean Christopherson <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]

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