mm/pkeys: generate pkey system call code only if ARCH_HAS_PKEYS is selected

Having code for the pkey_mprotect, pkey_alloc and pkey_free system calls
makes only sense if ARCH_HAS_PKEYS is selected.  If not selected these
system calls will always return -ENOSPC or -EINVAL.

To simplify things and have less code generate the pkey system call code
only if ARCH_HAS_PKEYS is selected.

For architectures which have already wired up the system calls, but do
not select ARCH_HAS_PKEYS this will result in less generated code and a
different return code: the three system calls will now always return
-ENOSYS, using the cond_syscall mechanism.

For architectures which have not wired up the system calls less
unreachable code will be generated.

Link: http://lkml.kernel.org/r/20161114111251.70084-1-heiko.carstens@de.ibm.com
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>