arm64: hibernate: use get_safe_page directly

create_safe_exec_page() uses hibernate's allocator to create a set of page
table to map a single page that will contain the relocation code.

Remove the allocator related arguments, and use get_safe_page directly, as
it is done in other local functions in this file to simplify function
prototype.

Removing this function pointer makes it easier to refactor the code later.

Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Will Deacon <will@kernel.org>