tools: bpftool: don't use hex numbers in JSON output

JSON does not accept hex numbers with 0x prefix.  Simply print
as decimal numbers, JSON should be primarily machine-readable.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Fixes: 831a0aaf ("tools: bpftool: add JSON output for `bpftool map *` commands")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>