mm: provide a fallback for PAGE_KERNEL_EXEC for architectures

mm: provide a fallback for PAGE_KERNEL_EXEC for architectures

Some architectures just don't have PAGE_KERNEL_EXEC.  The mm/nommu.c and
mm/vmalloc.c code have been using PAGE_KERNEL as a fallback for years.
Move this fallback to asm-generic.

Link: http://lkml.kernel.org/r/20180510185507.2439-3-mcgrof@kernel.org
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>