MFC: r220928
Remove the nm_mtx mutex locking from the test for
nm_maxfilesize. This value rarely, if ever, changes
and the nm_mtx mutex is locked/unlocked earlier in
the function, which should be sufficient to avoid
getting a stale cached value for it. There is a
discussion w.r.t. what these tests should be, but
I've left them basically the same as the regular
NFS client for now.