Fix the race while waiting for coveredvp lock during unmount. The vnode may be recycled during the sleep, wrap the vn_lock with vhold/vdrop. Check that coveredvp still points to the same mp after sleep (needed because sleep dropped Giant). Move check for user rights for unmount after coveredvp lock is obtained.
Fix the race while waiting for coveredvp lock during unmount. The vnode may
be recycled during the sleep, wrap the vn_lock with vhold/vdrop.
Check that coveredvp still points to the same mp after sleep (needed
because sleep dropped Giant).
Move check for user rights for unmount after coveredvp lock is obtained.

Tested by:	Peter Holm
Reviewed by:	tegge
Approved by:	kan (mentor)
MFC after:	2 weeks

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