Replace all mtx_lock()/mtx_unlock() on the iod lock with macros.

Since the NFS node mutex needs to change to an sx lock so it can be held when
vnode_pager_setsize() is called and the iod lock is held when the NFS node lock
is acquired, the iod mutex will need to be changed to an sx lock as well.
To simply the future commit that changes both the NFS node lock and iod lock
to sx locks, this commit replaces all mtx_lock()/mtx_unlock() calls on the
iod lock with macros.
There is no semantic change as a result of this commit.

I don't know when the future commit will happen and be MFC'd, so I have
set the MFC on this commit to one week so that it can be MFC'd at the same
time.

Suggested by: kib
MFC after: 1 week