FIX: two thread booleans to dm_safe_io corrected
FIX: ctr comment
Impl blockup on I/O error

Retry mechanism is preserved but
it immediately blocks up the whole system
if any device returns -EIO.

after block up
sysadmin must checkup the cause
and turn blockup switch to 0 to restart the system.
FIX: blockup: only sleep when -ENOMEM
Merge branch 'feature/mikulas-blockup-on-error' of https://github.com/akiradeveloper/dm-writeboost into feature/mikulas-blockup-on-error

Conflicts:
	Driver/dm-writeboost-daemon.c
	Driver/dm-writeboost.h