Drop the repeated word "struct" in a comment.

Link: https://lore.kernel.org/r/20200720001455.31882-1-rdunlap@infradead.orgSigned-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jan Kara <jack@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>