nullfs changes MFC

[Konstantin Belousov]

Hi,
I am going to merge latest batch of the nullfs improvements into
stable/9. This will bring up significant performance enchancements due
to use of the shared locks for lookups if the lower layer supports it,
much better caching on the nullfs layer, and proper handling of the text
segments on the nullfs. Also, it should improve the error recovery and
some corner cases with locking.

Unfortunately, the merge would break KBI for VFS, since it needs 5 new
VOP slots, and only three spares are left. We already are very liberal
with the VFS KBI, so I do not feel that the merge is not acceptable, due
to the benefits it brings to the nullfs.

The merge is available at 
http://people.freebsd.org/~kib/misc/nullfs_9.1.patch
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