selftest: move seek_to_smaps_entry() out of mlock2-tests.c

Function seek_to_smaps_entry() can be useful for other selftest
functionalities, so move it out to header file.

Link: http://lkml.kernel.org/r/1473325970-11393-3-git-send-email-wei.guo.simon@gmail.com
Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Eric B Munson <emunson@akamai.com>
Cc: Simon Guo <wei.guo.simon@gmail.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Alexey Klimov <klimov.linux@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>