ext2: Fix kernel-doc warnings

Document a few parameters of ext2_alloc_blocks().  Redo the
alloc_new_reservation() and find_next_reservable_window() kernel-doc
entirely.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230818201121.2720451-1-willy@infradead.org>