Use pin reference instead of LRU reference on export root and junction cache inodes

In order for cache_inode_kill_entry to work properly if an export root
or junction inode is deleted in the underlying file system, we must
pin the entries instead of taking an LRU reference.

Proper cleanup will be in the next patch.

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>