staging: writeboost: Don't free flush_job in flush_proc

Freeing the work_struct in the worker function causes page fault
when it reschedules after quitting the worker function.

This patch fixes this issue by embedding flush_job in rambuffer struct.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
staging: writeboost: Add read caching

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
add hash table

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>