LU-15082 osp: invalidate statfs data from the timer callback

LU-15082 osp: invalidate statfs data from the timer callback

osp_statfs_timer_cb() can be called just before statfs data gets
stale. this in turn may cause early wakeup to the precreate thread
which would find statfs data still up-to-data and go back to slepp.
if no precreate happens to this OSP (e.g. due to current space
usage), then the precreate thread will stay asleep for a long time,
statfs data won't get refreshed and this may block new objects
on the corresponding OST.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I86e16eed6f1068702db696a9ddec7a22994180b7
Reviewed-on: https://review.whamcloud.com/45199
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>