fs/mpage.c: factor clean_buffers() out of __mpage_writepage()

__mpage_writepage() is over 200 lines long, has 20 local variables, four
goto labels and could desperately use simplification.  Splitting
clean_buffers() into a helper function improves matters a little,
removing 20+ lines from it.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Dheeraj Reddy <dheeraj.reddy@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>