in a row, and we need to try to read the next block, and have passed a
non-NULL cookie pointer to VOP_READDIR, ensure that we free the cookie
buffer before re-doing VOP_READDIR, so that we don't leak memory.
This fix is similar to nfs_serv.c revisions 1.115 + 1.124.
This should fix the long-standing problem observed by e.g. using Linux-
emulated programs to take backup of servers, which is one of the problems
which were reported in PR#42661.
Thanks to pooka@ for the hints for traversing the VOP* layer.
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