HAMMER VFS - Make all entry points MPSAFE, remove giant & critical sections

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.