samples/vfs: fix printf format string for size_t

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 <arnd@arndb.de>
Link: https://lore.kernel.org/r/20250224141406.1400864-1-arnd@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>