Staging: zram: Refactor zram_read/write() functions

This patch refactor the code of zram_read/write() functions. It does
not removes a lot of duplicate code alone, but is mostly a helper for
the third patch of this series (Staging: zram: allow partial page
operations).
Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>