powerpc/32s: Prepare prevent_user_access() for user_access_end()

In preparation of implementing user_access_begin and friends
on powerpc, the book3s/32 version of prevent_user_access() need
to be prepared for user_access_end().

user_access_end() doesn't provide the address and size which
were passed to user_access_begin(), required by prevent_user_access()
to know which segment to modify.

The list of segments which where unprotected by allow_user_access()
are available in current->kuap. But we don't want prevent_user_access()
to read this all the time, especially everytime it is 0 (for instance
because the access was not a write access).

Implement a special direction named KUAP_CURRENT. In this case only,
the addr and end are retrieved from current->kuap.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/55bcc1f25d8200892a31f67a0b024ff3b816c3cc.1579866752.git.christophe.leroy@c-s.fr

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