UPSTREAM: arm64: compat: Refactor aarch32_alloc_vdso_pages()

aarch32_alloc_vdso_pages() needs to be refactored to make it
easier to disable kuser helpers.

Divide the function in aarch32_alloc_kuser_vdso_page() and
aarch32_alloc_sigreturn_vdso_page().

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
[will: Inlined sigpage allocation to simplify error paths]
Signed-off-by: Will Deacon <will.deacon@arm.com>
(cherry picked from commit 1255a7341bee6cd96df056ed0c13e44b59023687)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 154668398
Change-Id: Ia54bc0f0c0ae34b863f46df0b15b46657f934e81