unix: factor `unp_vnode_peer()` out of `unp_connectat()`

[inquire_JohnEricson.me]

Summary

Move the "resolve a locked vnode to the referenced peer socket it names"
block into a helper. Pure code motion: the caller now calls
unp_vnode_peer() and keeps the vput()/connect/sorele() sequence.

No functional change intended.

Note: This refactor isn't really necessary as unp_vnode_peer() will
only be called once throughout this entire patch series. I am just
including it out of my personal preferences for decomposing tasks into
smaller functions --- we can skip this patch if the reviewers don't like
this.

Signed-off-by: John Ericson <[e-mail]>
Assisted-by: Claude Code (Claude Opus 4.8 and Fable 5)

[markj]

sys/kern/uipc_usrreq.c

3043

These comments (here and elsewhere in the series) are somewhat verbose for my taste, they're just restating what the code does (or isn't doing).