Add CTLFLAG_STATS to some vfs sysctl OIDs

Add CTLFLAG_STATS to the following OIDs:

vfs.altbufferflushes
vfs.recursiveflushes
vfs.barrierwrites
vfs.flushwithdeps
vfs.reassignbufcalls

Refer to r353111.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon