vfs: prefix lookup and relookup with vfs_

[mjg]

Summary

"lookup" is too general of a name and already clashes with other symbols.

No functional changes.

I have some work which refactors the routine and moves parts out of it, need a better name than "lookup".

[imp]

lookup is indeed too generic a name.

[mckusick]

I picked those names when ufs was the only filesystem in BSD. I should have renamed them when I added the VFS interface. Better late than never!