Clean up export on release of last reference.

Also, take a sentinel reference when creating an export.

remove_gsh_export will remove this sentinel reference.

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>
First pass of pseudo_unmount_export

Limitations:

Does not handle situation where junction cache inode is killed.

Does not remove FSAL_PSEUDO directories leading only to the export.

Does not clean out the directories in an FSAL_PSEUDO export that is
removed.

Does not handle unmount of any exports mounted on the removed export.

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>
Provide unexport and kill_export_root_entry routines

unexport will be called by admin driven unexport.

kill_export_root_entry is called by cache inode after the last LRU
reference to a killed entry has been removed. It will clean up the
no longer viable export.

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