Merge the 32bit and 64bit version.

Halve the check constants on 32bit.

Use STACK_TOP since it is defined.

Passing is_64 is now redundant since is_32bit_task() is used to
determine which callchain variant should be used. Use STACK_TOP and
is_32bit_task() directly.

This removes a page from the valid 32bit area on 64bit:
 #define TASK_SIZE_USER32 (0x0000000100000000UL - (1 * PAGE_SIZE))
 #define STACK_TOP_USER32 TASK_SIZE_USER32

Change return value to bool. It is inverted by users anyway.

Change to invalid_user_sp to avoid inverting the return value twice.
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/be8e40fc0737fb28ad08b198552dee7cac1c5ce2.1584699455.git.msuchanek@suse.de