Replace all mtx_assert() calls for n_mtx and ncl_iod_mutex with macros.

To be consistent with replacing the mtx_lock()/mtx_unlock() calls on
the NFS node mutex (n_mtx) and ncl_iod_mutex, this patch replaces
all mtx_assert() calls on these mutexes with macros as well.
This will simplify changing these locks to sx locks in a future commit.
However, this change may be delayed indefinitely, since it appears there
is a deadlock when vnode_pager_setsize() is called to shrink the size
and the NFS node lock is held.
There is no semantic change as a result of this commit.

Suggested by: kib
MFC after: 1 week