selftests/mm/ksm_functional: prevent unmapping undefined address

selftests/mm/ksm_functional: prevent unmapping undefined address

Replace some goto statements with return statements so that unmap() is not
called on an undefined address.  This change is made so that unmap() can
only be reached after mmap() is called (and the address mentioned is
defined).  Returning MAP_FAILED seems acceptable since client code checks
for this value.

Link: https://lkml.kernel.org/r/20240105202401.28851-1-inwardvessel@gmail.com
Fixes: 42096aa24b82 ("selftest/mm: ksm_functional_tests: test in mmap_and_merge_range() if anything got merged")
Signed-off-by: JP Kobryn <inwardvessel@gmail.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>