HAMMER VFS - Make all entry points MPSAFE, remove giant & critical sections
* All VFS, VOP, ioops, and bio_done entry points are now mpsafe and no
longer use giant.
* Implement hmp->fs_token and hmp->io_token for each HAMMER mount.
All operations that previously needed the MP lock now use hmp->fs_token.
All operations that interact with BIO callbacks now use hmp->io_token.
All critical sections now use io_token (these previously interlocked
against IO callbacks).
NOTE: read (for cached data) and getattr were MPSAFE before and
continue to be MPSAFE.