(lookup 4) ufs: add support for lockless lookup

[mjg]

Summary

ACLs are not supported, meaning their presence will force the use of the old lookup.

[mckusick]

I cannot speak to the correctness of the whole patch, but looking over D25577, D25578, and this one, D25579, I see nothing that looks incorrect.

[mjg]

• rebase

[mjg]

• add missing seqc to ffs_write for case of clearing suid/sgid

[kib]

sys/ufs/ffs/ffs_vnops.c

790

I am not sure why clearing setuid/setgid bits on normal files (only normal files can be written to) requires blocking of lockless lookup. The bits do not affect permissions to do the lookup.

1088

Same.

sys/ufs/ufs/ufs_vnops.c

1626

is it _begin() or _end() ?

[mjg]

• fix mismatched seqc on rename error

[mjg]

sys/ufs/ffs/ffs_vnops.c

790

This is not necessary. It alters i_mode and I think only modifying it within seqc is a good invariant to have. However, this can be pushed way down to wrap the actual modification, I have no strong opinion.

[mjg]

• tighten seqc coverage in write/extwrite

[kib]

In D25579#570728, @mjg wrote:

• tighten seqc coverage in write/extwrite

I think even this reduced scope is not needed.