LU-14125 osc: prevent overflow of o_dropped

LU-14125 osc: prevent overflow of o_dropped

In osc_announce_cached(), prevent o_dropped from overflowing.
Necessary because o_dropped AKA o_misc is 32 bits, but cl_lost_grant
is 64 bits.

Add a CDEBUG call so we can tell whether this happened.

Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Change-Id: Ia459934c789ae9609f851ae0a2581de583c6fc1c
Reviewed-on: https://review.whamcloud.com/40659
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>