Read lock allows to retrieve informations about flushing progress and printing
progress bar during changing cache mode.
Flushing dirty data during changing cache mode is done twice - first flush might
be interrupted by user and the second one, called with write lock acquired, is
uninterruptable.
Signed-off-by: Michal Mielewczyk <michal.mielewczyk@intel.com>
dca9769298