MFC: r352636
Replace all mtx_lock()/mtx_unlock() on n_mtx with the macros.

For a long time, some places in the NFS code have locked/unlocked the
NFS node lock with the macros NFSLOCKNODE()/NFSUNLOCKNODE() whereas
others have simply used mtx_lock()/mtx_unlock().
Since the NFS node mutex needs to change to an sx lock so it can be held when
vnode_pager_setsize() is called, replace all occurrences of mtx_lock/mtx_unlock
with the macros to simply making the change to an sx lock in future commit.
There is no semantic change as a result of this commit.