exofs: Fix double page_unlock BUG in write_begin/end

This BUG is there since the first submit of the code, but only triggered
in last Kernel. It's timing related do to the asynchronous object-creation
behaviour of exofs. (Which should be investigated farther)

The bug is obvious hence the fixed.

Signed-off-by: Boaz Harrosh &lt;Boaz Harrosh <bharrosh@panasas.com>