libafs: consistently hold vnode refs

libafs: consistently hold vnode refs

Make all common libafs code call AFS_FAST_HOLD to hold an afs vnode
reference, instead of sometimes calling VN_HOLD(AFSTOV(tvc)) directly.
Make AFS_FAST_HOLD always call osi_vnhold, and have each platform
define osi_vnhold for their platform-specific quirks (as it was doing
before).

This way, if a platform has an osi_vnhold that is special somehow (on
solaris, we bump a VFS refcount if the vnode refcount goes from 0->1),
it will always be called on vnode references.

Change-Id: I7bb2b5ac116c8e86f6c6c0d40f414523870275a3
Reviewed-on: http://gerrit.openafs.org/1987
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>