p9fs: Set vnode state in p9fs_vget_common()

Once a new vnode is visible from the mountpoint hash, we should set its
state from VSTATE_UNINITIALIZED to VSTATE_CONSTRUCTED. I do not think
this affects correctness at all, but the bug trips a check in
vop_unlock_debugpost(), previously hidden under options DEBUG_VFS_LOCKS.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51720