Don't set flags on the mount structure before all permission checks have
been done.

Don't allow multiple mount operations with MNT_UPDATE at the same
time on the same mount point. When the first mount operation
completed, MNT_UPDATE was cleared in the mount structure, causing
the second to complete as if it was a no-update mount operation
with the following bad side effects:

• mount structure inserted multiple times onto the mountlist
• vp->v_mountedhere incorrectly set, causing next namei operation walking into the mountpoint to crash with a locking against myself panic.

Plug a vnode leak in case vinvalbuf fails.