khugepaged: rename collapse_shmem() and khugepaged_scan_shmem()

Next patch will add khugepaged support of non-shmem files.  This patch
renames these two functions to reflect the new functionality:

    collapse_shmem()        =>  collapse_file()
    khugepaged_scan_shmem() =>  khugepaged_scan_file()

Link: http://lkml.kernel.org/r/20190801184244.3169074-6-songliubraving@fb.com


Signed-off-by: Song Liu <songliubraving@fb.com>
Acked-by: Rik van Riel <riel@surriel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>