mm: refactor swap-in logic out of shmem_getpage_gfp

swapin logic can be reused independently without rest of the logic in
shmem_getpage_gfp.  So lets refactor it out as an independent function.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Vineeth Remanan Pillai <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Kelley Nielsen <[email protected]>
Cc: Rik van Riel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>