nullfs: Apply comments from D38761 (style fixes, small improvements)

[olce]

Summary

In two commits:

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.

MFC with:       249ec85352b5 ("nullfs: smr-protected hash lookup")
Sponsored by:   The FreeBSD Foundation

nullfs: Slightly reduce contention by reducing concurrent sections

In null_lock_prep_with_smr(), initialize 'lvp' outside of the
SMR-protected section.

In null_lock(), if after locking the lower vnode we notice that we have
been reclaimed, we have to unlock the lower vnode and then relock our
own now that the lock isn't shared anymore.  Call VOP_UNLOCK() on the
lower vnode as soon as this condition is known.

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

MFC with:       641a58239520 ("nullfs: avoid the interlock in null_lock with smr")
Sponsored by:   The FreeBSD Foundation

[kib]

sys/fs/nullfs/null_subr.c

39

Can be rwlock.h moved?

[olce]

sys/fs/nullfs/null_subr.c

39

Sure. Missed this pre-existing problem.