vfs: Fix vop_stdis_text()

atomic(9) primitives are documented as operating on unsigned types.
Here, we need a cast to avoid a tautological comparison.

Add a regression test for access(2), which was affected by the bug.

Reported by: NetApp
Reviewed by: kib
Fixes: e511bd1406fa ("vfs: fully lockless v_writecount adjustment")
MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D47672