nullfs: Fix some style after recent changes

In 'nullfs_subr.c':

• Sort the headers.
• Use 'func' in preference to the function's name in an assertion.

This applies comments from D38761, one of which was missed and the other
added too late.

Reviewed by: kib
MFC with: 249ec85352b5 ("nullfs: smr-protected hash lookup")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52935