FBSD TryToSmush locking fixup

FBSD TryToSmush locking fixup

We need to hold the interlock when we check v_flags, so do so.

TryToSmush is sometimes called with the vnode already locked,
as the current code appears to hold the vnode lock for the
current working directory (which is probably a bug).  Check if
the lock is already held (panic if someone else has it!), and
unlock as appropriate when we're done.

Change-Id: Id09ef1e10632c7f63b590271a6339a069176deac
Reviewed-on: http://gerrit.openafs.org/2294
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>