unionfs: implement VOP_UNP_* and remove special VSOCK vnode handling

[jah]

Summary

unionfs has a bunch of clunky special-case code to avoid creating
unionfs wrapper vnodes for AF_UNIX sockets. This was added in 2008
to address PR 118346, but in the intervening years the VOP_UNP_*
operations have been added to provide a clean interface to allow
sockets to work in the presence of stacked filesystems.

PR: 275871
Tested by: Karlo Miličević <[e-mail]>
MFC after: 2 weeks

[olce]

Great! More cruft removed, including one call to unionfs_relookup(). I have a few suggestions.

sys/fs/unionfs/union_vnops.c

1044–1046

I would now assert that a_vp is also a unionfs node.

1068–1069

unp is never supposed to be NULL now here.

1071–1072

Same here.

[jah]

Code review feedback, also remove a nonsensical check from unionfs_link()