- Remove GIANT_REQUIRED where giant is no longer required.

 - Remove GIANT_REQUIRED where giant is no longer required.
 - Protect access to mnt_kern_flag with the mountpoint mutex.
 - Use the appropriate nd flags to deal with giant in vn_open_cred().
   We currently determine whether the caller is mpsafe by checking
   for a valid fdidx.  Any caller coming from user-space is now
   mpsafe and supplies a valid fd.  No kenrel callers have been
   converted to mpsafe, so this check is sufficient for now.
 - Use VFS_LOCK_GIANT instead of manual giant acquisition where
   appropriate.

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