write_inode_now(): avoid unnecessary synchronous write

We shouldn't use WB_SYNC_ALL if the caller is asking for asynchronous
treatment.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>