shmem: fix tmpfs reconfiguration (remount) when noswap is set

In systemd we're trying to switch the internal credentials setup logic
to new mount API [1], and I noticed fsconfig(FSCONFIG_CMD_RECONFIGURE)
consistently fails on tmpfs with noswap option. This can be trivially
reproduced with the following:

```
int fs_fd = fsopen("tmpfs", 0);
fsconfig(fs_fd, FSCONFIG_SET_FLAG, "noswap", NULL, 0);
fsconfig(fs_fd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
fsmount(fs_fd, 0, 0);
fsconfig(fs_fd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);  <------ EINVAL
```

After some digging the culprit is shmem_reconfigure() rejecting
!(ctx->seen & SHMEM_SEEN_NOSWAP) && sbinfo->noswap, which is bogus
as ctx->seen serves as a mask for whether certain options are touched
at all. On top of that, noswap option doesn't use fsparam_flag_no,
hence it's not really possible to "reenable" swap to begin with.
Drop the check and redundant SHMEM_SEEN_NOSWAP flag.

[1] systemd/systemd#39637

Fixes: 2c6efe9 ("shmem: add support to ignore swap")
Signed-off-by: Mike Yuan <me@yhndnzj.com>
Link: https://patch.msgid.link/20251108190930.440685-1-me@yhndnzj.com
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: YHNdnzj

mm/shmem.c

@@ -131,8 +131,7 @@ struct shmem_options {
 #define SHMEM_SEEN_INODES 2
 #define SHMEM_SEEN_HUGE 4
 #define SHMEM_SEEN_INUMS 8
-#define SHMEM_SEEN_NOSWAP 16
-#define SHMEM_SEEN_QUOTA 32
+#define SHMEM_SEEN_QUOTA 16
 };
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
@@ -4677,7 +4676,6 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param)
 				       "Turning off swap in unprivileged tmpfs mounts unsupported");
 		}
 		ctx->noswap = true;
-		ctx->seen |= SHMEM_SEEN_NOSWAP;
 		break;
 	case Opt_quota:
 		if (fc->user_ns != &init_user_ns)
@@ -4827,14 +4825,15 @@ static int shmem_reconfigure(struct fs_context *fc)
 		err = "Current inum too high to switch to 32-bit inums";
 		goto out;
 	}
-	if ((ctx->seen & SHMEM_SEEN_NOSWAP) && ctx->noswap && !sbinfo->noswap) {
+
+	/*
+	 * "noswap" doesn't use fsparam_flag_no, i.e. there's no "swap"
+	 * counterpart for (re-)enabling swap.
+	 */
+	if (ctx->noswap && !sbinfo->noswap) {
 		err = "Cannot disable swap on remount";
 		goto out;
 	}
-	if (!(ctx->seen & SHMEM_SEEN_NOSWAP) && !ctx->noswap && sbinfo->noswap) {
-		err = "Cannot enable swap on remount if it was disabled on first mount";
-		goto out;
-	}
 
 	if (ctx->seen & SHMEM_SEEN_QUOTA &&
 	    !sb_any_quota_loaded(fc->root->d_sb)) {