The recent NFS forced unmount improvements introduced a side-effect

The recent NFS forced unmount improvements introduced a side-effect
where some client operations might be unexpectedly cancelled during
an unsuccessful non-forced unmount attempt. This causes problems
for amd(8), because it periodically attempts a non-forced unmount
to check if the filesystem is still in use.

Fix this by adding a new mountpoint flag MNTK_UNMOUNTF that is set
only during the operation of a forced unmount. Use this instead of
MNTK_UNMOUNT to trigger the cancellation of hung NFS operations.

Also correct a problem where dounmount() might inadvertently clear
the MNTK_UNMOUNT flag.

Reported by:	simokawa
MFC after:	1 week