exofs: Cleaup read path in regard with read_for_write

Last BUG fix added a flag to the the page_collect structure
to communicate with readpage_strip. This calls for a clean up
removing that flag's reincarnations in the read functions
parameters.

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