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>