bpf: Use refcount_t instead of atomic_t for mmap_count

bpf: Use refcount_t instead of atomic_t for mmap_count

Use an API that resembles more the actual use of mmap_count.

Found by cocci:
kernel/bpf/arena.c:245:6-25: WARNING: atomic_dec_and_test variation before object free at line 249.

Fixes: b90d77e5fd78 ("bpf: Fix remap of arena.")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202412292037.LXlYSHKl-lkp@intel.com/
Signed-off-by: Pei Xiao <xiaopei01@kylinos.cn>
Link: https://lore.kernel.org/r/6ecce439a6bc81adb85d5080908ea8959b792a50.1735542814.git.xiaopei01@kylinos.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>