VFS: export sync_sb_inodes

This patch exports the 'sync_sb_inodes()' which is needed for
UBIFS because it has to force write-back from time to time.
Namely, the UBIFS budgeting subsystem forces write-back when
its pessimistic callculations show that there is no free
space on the media.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>