linux: support AT_EMPTY_PATH flag in fchownat(2)

This fixes rsyslog package installation scripts in Bionic.

Reviewed By: kib
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D29108

(cherry picked from commit e47823b8312b7cb61c5c8cb8e1eb981994e3a0b4)