nfscl: Fix must_commit/writeverf handling for Direct I/O

Without this patch, the KASSERT(must_commit == 0,..) can be
triggered by the writeverf in the Direct I/O write reply changing.
This is not a situation that should cause a panic(). Correct
handling is to ignore the change in "writeverf" for Direct
I/O, since it is done with NFSWRITE_FILESYNC.

This patch modifies the semantics of the "must_commit"
argument slightly, allowing an initial value of 2 to indicate
that a change in "writeverf" should be ignored.
It also fixes the KASSERT()s.

This bug would affect few, since Direct I/O is not enabled
by default and "writeverf" rarely changes. Normally "writeverf"
only changes when a NFS server reboots, however I found the
bug when testing against a Linux 5.15.1 kernel nfsd, which
replied to a NFSWRITE_FILESYNC write with a "writeverf" of all
0x0 bytes.

MFC after: 2 weeks