mm: allow arch code to control the user page table ceiling

mm: allow arch code to control the user page table ceiling

On architectures where a pgd entry may be shared between user and kernel
(e.g.  ARM+LPAE), freeing page tables needs a ceiling other than 0.
This patch introduces a generic USER_PGTABLES_CEILING that arch code can
override.  It is the responsibility of the arch code setting the ceiling
to ensure the complete freeing of the page tables (usually in
pgd_free()).

[[email protected]: commit log; shift_arg_pages(), asm-generic/pgtables.h changes]
Signed-off-by: Hugh Dickins <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>
Cc: Russell King <[email protected]>
Cc: <[email protected]>	[3.3+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>