FBSD try-relax child vnode locking (recurse only)

FBSD  try-relax child vnode locking (recurse only)

In cases where afs_vop_lookup would return a child vnode
locked, continue to take an exclusive lock, but allow
recursion (LK_CANRECURSE).  Allow recursion also at
afs_vop_link, where we specifically encountered a conflict
due to recursion.

Change-Id: I5f5b0979ce3c86e517f96ea60b96ebdd26401acb
Reviewed-on: http://gerrit.openafs.org/2625
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit ec85304dd738935d8be3f8934b166f78a10995bf)
Reviewed-on: http://gerrit.openafs.org/2683