samples/vfs/mountinfo: Use __u64 instead of uint64_t

samples/vfs/mountinfo: Use __u64 instead of uint64_t

On 32-bit (e.g. arm32, m68k):

    samples/vfs/mountinfo.c: In function ‘dump_mountinfo’:
    samples/vfs/mountinfo.c:145:29: warning: format ‘%lx’ expects argument of type ‘long unsigned int’, but argument 2 has type ‘uint64_t’ {aka ‘long long unsigned int’} [-Wformat=]
      145 |                 printf("0x%lx 0x%lx 0x%llx ", mnt_ns_id, mnt_id, buf->mnt_parent_id);
	  |                           ~~^                 ~~~~~~~~~
	  |                             |                 |
	  |                             long unsigned int uint64_t {aka long long unsigned int}
	  |                           %llx
    samples/vfs/mountinfo.c:145:35: warning: format ‘%lx’ expects argument of type ‘long unsigned int’, but argument 3 has type ‘uint64_t’ {aka ‘long long unsigned int’} [-Wformat=]
      145 |                 printf("0x%lx 0x%lx 0x%llx ", mnt_ns_id, mnt_id, buf->mnt_parent_id);
	  |                                 ~~^                      ~~~~~~
	  |                                   |                      |
	  |                                   long unsigned int      uint64_t {aka long long unsigned int}
	  |                                 %llx

Just using "%llx" instead of "%lx" is not sufficient, as uint64_t is
"long unsigned int" on some 64-bit platforms like arm64.  Hence also
replace "uint64_t" by "__u64", which matches what most other samples
are already using.

Fixes: d95e49bf8bcdc7c1 ("samples: add a mountinfo program to demonstrate statmount()/listmount()")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20250106134802.1019911-1-geert+renesas@glider.be
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>