ubifs: fix bogus __mark_inode_dirty(I_DIRTY_SYNC | I_DIRTY_DATASYNC) call

I_DIRTY_DATASYNC is a strict superset of I_DIRTY_SYNC semantics, as
in mark dirty to be written out by fdatasync as well.  So dirtying
for both flags makes no sense.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>