to prevent a deadlock trying to call VOP_PUTPAGES() on a VDIROP vnode.
This can happen when a stacked filesystem is mounted on top of an LFS: an
LFS dirop needs to get a vnode, which is available from the upper layer.
The corresponding lower layer vnode, however, is VDIROP, so the upper layer
can't be cleaned out since its VOP_PUTPAGES() is passed through to the lower
layer, which waits for dirops to drain before it can proceed. Deadlock.
Tweak ufs_makeinode() and ufs_mkdir() to pass the a_vpp argument through
to VOP_VALLOC().
Partially addresses PR # 26043, though it probably does not completely fix
the problem described there.
c716c3d307