Revert r184118. There is actually a code in the kernel, for instance in
kern_unlinkat(), that expects that vn_start_write() actually fills the mp
even when the call failed.

As Tor noted, that pattern relies on the the type stability of the mount
points, as well as that suspended mount points are never freed and
V_XSLEEP is always passed to vn_start_write() when called on a freed
mount point.

Reported by: stass
Reviewed by: tegge
PR: 123768