Move checks and accounting updates from __delete_from_page_cache() into
a separate function.  We will reuse it when batching page cache
truncation operations.

Link: http://lkml.kernel.org/r/20171010151937.26984-7-jack@suse.czSigned-off-by: Jan Kara <jack@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>