pipe: reduce atime precision

The routine is called on successful write and read, which on pipes happens a
lot and for small sizes.

Precision provided by default seems way bigger than necessary and it causes
problems in vms on amd64 (it rdtscp's which vmexits). getnanotime seems to
provide the level roughly in lines of Linux so we should be good here.

Sample result from will-it-scale pipe1_processes -t 1 (ops/s):
before: 426464
after: 3247421

Note the that atime handling for named pipes is broken with and without the
patch. The filesystem code is never used for updating atime and never looks
at the updated field. Consequently, while there are no provisions added to
handle named pipes separately, the change is a nop for that case.

Differential Revision: https://reviews.freebsd.org/D23964