Use deleg_list for only REGULAR_FILE objects

nfs4_op_delegreturn() should check to make sure that it is a regular
file before using deleg_list as it is not initialized for any other file
type.  Also changed a check for non DIRECTORY to REGULAR_FILE in
do_delegation().

Signed-off-by: Malahal Naineni <malahal@us.ibm.com>