osd: cast `whoami` to unsigned so it can be used as the seed for RNG

osd: cast `whoami` to unsigned so it can be used as the seed for RNG

default_random_engine's result_type is `unsigned int`, so we need to
pass an `unsigned int` as its seed.

Fixes: http://tracker.ceph.com/issues/26890
Signed-off-by: Kefu Chai <kchai@redhat.com>
Conflicts:
	src/osd/OSD.cc: this breaks the build with clang. and in master
we are not using std::default_random_engine for setting the scrub
interval. so this change is not cherry-picked from master.