[issue1841] vfscache panic when creating many links

[Matthew Dillon]

:If we allow cache_alloc to return null, we could modify the lookup path to call 
:cache_cleanneg() to try to get back some space, at least from negative entries, 
    Negative hits aren't the problem.  Having all the ncp's associated with
    just a few vnodes means the normal vnode pressure will not clear them
    out, because the vnodes are not under pressure.

    We don't want the allocation in the loop to fail, we want it to
    clean out some ncps prior to entering the loop I think.

					    -Matt