powerpc/pkeys: make protection key 0 less special

powerpc/pkeys: make protection key 0 less special

Applications need the ability to associate an address-range with some
key and latter revert to its initial default key. Pkey-0 comes close to
providing this function but falls short, because the current
implementation disallows applications to explicitly associate pkey-0 to
the address range.

Lets make pkey-0 less special and treat it almost like any other key.
Thus it can be explicitly associated with any address range, and can be
freed. This gives the application more flexibility and power.  The
ability to free pkey-0 must be used responsibily, since pkey-0 is
associated with almost all address-range by default.

Even with this change pkey-0 continues to be slightly more special
from the following point of view.
(a) it is implicitly allocated.
(b) it is the default key assigned to any address-range.
(c) its permissions cannot be modified by userspace.

NOTE: (c) is specific to powerpc only. pkey-0 is associated by default
with all pages including kernel pages, and pkeys are also active in
kernel mode. If any permission is denied on pkey-0, the kernel running
in the context of the application will be unable to operate.

Tested on powerpc.

Signed-off-by: Ram Pai <linuxram@us.ibm.com>
[mpe: Drop #define PKEY_0 0 in favour of plain old 0]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>