samples/vfs: fix printf format string for size_t

size_t needs a %z format string modifier instead of %l

samples/vfs/test-list-all-mounts.c:152:39: warning: format specifies type 'unsigned long' but the argument has type 'size_t' (aka 'unsigned int') [-Wformat]
152 | printf("mnt_uidmap[%lu]:\t%s\n", idx, idmap);
| ~~~ ^~~
| %zu
samples/vfs/test-list-all-mounts.c:161:39: warning: format specifies type 'unsigned long' but the argument has type 'size_t' (aka 'unsigned int') [-Wformat]
161 | printf("mnt_gidmap[%lu]:\t%s\n", idx, idmap);
| ~~~ ^~~
| %zu

Fixes: fa204a65f1b6 ("samples/vfs: add STATMOUNT_MNT_{G,U}IDMAP")
Signed-off-by: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

samples/vfs/test-list-all-mounts.c

···
 				const char *idmap = stmnt->str + stmnt->mnt_uidmap;
 
 				for (size_t idx = 0; idx < stmnt->mnt_uidmap_num; idx++) {
-					printf("mnt_uidmap[%lu]:\t%s\n", idx, idmap);
+					printf("mnt_uidmap[%zu]:\t%s\n", idx, idmap);
 					idmap += strlen(idmap) + 1;
 				}
 			}
···
 				const char *idmap = stmnt->str + stmnt->mnt_gidmap;
 
 				for (size_t idx = 0; idx < stmnt->mnt_gidmap_num; idx++) {
-					printf("mnt_gidmap[%lu]:\t%s\n", idx, idmap);
+					printf("mnt_gidmap[%zu]:\t%s\n", idx, idmap);
 					idmap += strlen(idmap) + 1;
 				}
 			}