UBIFS: do not commit twice

VFS calls '->sync_fs()' twice - first time with @wait = 0, second
time with @wait = 1. As a result, we may commit and synchronize
write-buffers twice. Avoid doing this by returning immediatelly if
@wait = 0.

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