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

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/[email protected]
Signed-off-by: Simon Guo <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Eric B Munson <[email protected]>
Cc: Simon Guo <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Alexey Klimov <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Thierry Reding <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>