libpagemap: make pm_kernel_* APIs consistently return negative errors.

libpagemap: make pm_kernel_* APIs consistently return negative errors.

pm_kernel_* APIs return errnos as error values, which works because
almost all callers check if the return value is 'true' or 'false'.
Most other APIs in libpagemap return negative errors however.

Fix pm_kernel_* APIs to be consistent with other library functions.

Bug: 111694435
Test: procrank; procmem 1; librank

Change-Id: Ia5cbf6a5676bc797f7cf0843f8b90228bfc3eaec
Signed-off-by: Sandeep Patil <sspatil@google.com>