NFSv4.2: Add a tracepoint for listxattr

This can be defined as simply an NFS4_INODE_EVENT() since we don't have
the name of a specific xattr to list. This roughly matches readdir,
which also uses an NFS4_INODE_EVENT() tracepoint.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>