STABLE14-macos-vnode-get-fixes-20060105

STABLE14-macos-vnode-get-fixes-20060105

The afs_pioctl.c change should fix a real crash (panic), but fs flushv isn't
that common an operation.
Other changes:
don't GUNLOCK() around vnode_get(). we weren't consistent about it, and it
doesn't appear to be strictly required.
handle vnode_get() failures in more cases
darwin_vn_hold will panic if vnode is terminating rather than mess up the
refcounts.

(cherry picked from commit 8b015a5e325b52969203d0647f67945ab7e402cc)