btrfs: use the correct const function attribute for btrfs_get_num_csums

The build robot reports

compiler: h8300-linux-gcc (GCC) 9.3.0
   In file included from fs/btrfs/tests/extent-map-tests.c:8:
>> fs/btrfs/tests/../ctree.h:2166:8: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
    2166 | size_t __const btrfs_get_num_csums(void);
         |        ^~~~~~~

The function attribute for const does not follow the expected scheme and
in this case is confused with a const type qualifier.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -68,7 +68,7 @@ const char *btrfs_super_csum_driver(u16 csum_type)
 		btrfs_csums[csum_type].name;
 }
 
-size_t __const btrfs_get_num_csums(void)
+size_t __attribute_const__ btrfs_get_num_csums(void)
 {
 	return ARRAY_SIZE(btrfs_csums);
 }

fs/btrfs/ctree.h

@@ -2262,7 +2262,7 @@ BTRFS_SETGET_STACK_FUNCS(super_uuid_tree_generation, struct btrfs_super_block,
 int btrfs_super_csum_size(const struct btrfs_super_block *s);
 const char *btrfs_super_csum_name(u16 csum_type);
 const char *btrfs_super_csum_driver(u16 csum_type);
-size_t __const btrfs_get_num_csums(void);
+size_t __attribute_const__ btrfs_get_num_csums(void);
 
 
 /*