g_vfs_done: Only report ENXIO once

The contract with the lower layers is that once ENXIO is reported, all
further I/O to the device is not possible. This is reported when the
device departs for good or changes in some material manner out from
underneath the system. Since the lower layers terminate all pending I/O
when this is detected with ENXIO, reporting more than one provides no
extra value. ENXIO suppression done with atomics due to race described
in e8827f4094cb. It's on the error path and a rare event, so this won't affect
performance.

Sponsored by: Netflix
Reviewed by: mckusick, kib
Differential Revision: https://reviews.freebsd.org/D35034