sbt is the time in the future that the tsleep_sbt() is expected to be completed
at. sbtt is the current time. Depending on the precision with sysctl
kern.timecounter.alloweddeviation the start time may be incremented by
tc_tick_sbt. The same increment is needed for the current time of sbtt before
calculating the difference. The impact of missing this increment is that rmtp
may increase by one tc_tick_sbt on every early [EINTR] return. If the same
struct is passed in for rqtp as rmtp this can result in rqtp effectively
incrementing by tc_tick_sbt and sleeping longer than originally intended.
This problem was introduced in r247797.
Reviewed by: kib, markj, vangyzen (all on an older version of the test)
MFC after: 2 weeks
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D14362
db51b6bdae