nilfs2: make superblock data array index computation sparse friendly

Upon running sparse, "warning: dubious: x & !y" is output at an array
index calculation within nilfs_load_super_block().

The calculation is not wrong, but to eliminate the sparse warning, replace
it with an equivalent calculation.

Also, add a comment to make it easier to understand what the unintuitive
array index calculation is doing and whether it's correct.

Link: https://lkml.kernel.org/r/20240430080019.4242-3-konishi.ryusuke@gmail.com
Fixes: e339ad3 ("nilfs2: introduce secondary super block")
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: kernel test robot <lkp@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: konis

fs/nilfs2/the_nilfs.c

@@ -592,7 +592,7 @@ static int nilfs_load_super_block(struct the_nilfs *nilfs,
 	struct nilfs_super_block **sbp = nilfs->ns_sbp;
 	struct buffer_head **sbh = nilfs->ns_sbh;
 	u64 sb2off, devsize = bdev_nr_bytes(nilfs->ns_bdev);
-	int valid[2], swp = 0;
+	int valid[2], swp = 0, older;
 
 	if (devsize < NILFS_SEG_MIN_BLOCKS * NILFS_MIN_BLOCK_SIZE + 4096) {
 		nilfs_err(sb, "device size too small");
@@ -648,9 +648,25 @@ static int nilfs_load_super_block(struct the_nilfs *nilfs,
 	if (swp)
 		nilfs_swap_super_block(nilfs);
 
+	/*
+	 * Calculate the array index of the older superblock data.
+	 * If one has been dropped, set index 0 pointing to the remaining one,
+	 * otherwise set index 1 pointing to the old one (including if both
+	 * are the same).
+	 *
+	 *  Divided case             valid[0]  valid[1]  swp  ->  older
+	 *  -------------------------------------------------------------
+	 *  Both SBs are invalid        0         0       N/A (Error)
+	 *  SB1 is invalid              0         1       1         0
+	 *  SB2 is invalid              1         0       0         0
+	 *  SB2 is newer                1         1       1         0
+	 *  SB2 is older or the same    1         1       0         1
+	 */
+	older = valid[1] ^ swp;
+
 	nilfs->ns_sbwcount = 0;
 	nilfs->ns_sbwtime = le64_to_cpu(sbp[0]->s_wtime);
-	nilfs->ns_prot_seq = le64_to_cpu(sbp[valid[1] & !swp]->s_last_seq);
+	nilfs->ns_prot_seq = le64_to_cpu(sbp[older]->s_last_seq);
 	*sbpp = sbp[0];
 	return 0;
 }