For each file in sysctl(KERN_FILE_BYFILE), FILLIT() calls fill_file(),

For each file in sysctl(KERN_FILE_BYFILE), FILLIT() calls fill_file(),
which calls VOP_GETATTR().  For NFS, that leads to nfs_getattr().
If the node's attributes are not in NFS's cache, nfs_getattr() will
invoke nfs_request() and the latter will sleep, allowing the file
pointer to disappear while we traverse the list.
This results in kernel crashes while running netstat or pstat -f.
Grab a reference to the file descriptor before calling FILLIT(),
and release it afterwards.  This way the file descriptor cannot
disappear while we sleep in nfs_getattr().
Analysis and fix from Pedro Martelletto; input and OK guenther@ mpi@