Fix a credential reference leak. [1]

Fix a credential reference leak. [1]

Close subtle but relatively unlikely race conditions when
propagating the vnode write error to other active sessions
tracing to the same vnode, without holding a reference on
the vnode anymore. [2]

PR:		kern/126368 [1]
Submitted by:	rwatson [2]
Reviewed by:	kib, rwatson
MFC after:	4 weeks