ext2: Fix some kernel-doc warnings in balloc.c

Fixes the following W=1 kernel build warning(s):

fs/ext2/balloc.c:203: warning: Excess function parameter 'rb_root' description in '__rsv_window_dump'
fs/ext2/balloc.c:294: warning: Excess function parameter 'rb_root' description in 'search_reserve_window'
fs/ext2/balloc.c:878: warning: Excess function parameter 'rsv' description in 'alloc_new_reservation'

Link: https://lore.kernel.org/r/20200911114036.60616-1-wanghai38@huawei.com
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>