Modify mpage_writepages to optionally only write back dirty pages within a
specified range in a file (as in the case of O_SYNC).  Cheat a little to avoid
changes to prototypes of aops - just put the <start, end> hint into the
writeback_control struct instead.  If <start, end> are not set, then default
to writing back all the mapping's dirty pages.
Signed-off-by: Suparna Bhattacharya <suparna@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>