samples/landlock: Fix port parsing in sandboxer

samples/landlock: Fix port parsing in sandboxer

If you want to specify that no port can be bind()ed, you would think
(looking quickly at both help message and code) that setting
LL_TCP_BIND="" would do it.

However the code splits on ":" then applies atoi(), which does not allow
checking for errors. Passing an empty string returns 0, which is
interpreted as "allow bind(0)", which means bind to any ephemeral port.
This bug occurs whenever passing an empty string or when leaving a
trailing/leading colon, making it impossible to completely deny bind().

To reproduce:
export LL_FS_RO="/" LL_FS_RW="" LL_TCP_BIND=""
./sandboxer strace -e bind nc -n -vvv -l -p 0
Executing the sandboxed command...
bind(3, {sa_family=AF_INET, sin_port=htons(0),
     sin_addr=inet_addr("0.0.0.0")}, 16) = 0
Listening on 0.0.0.0 37629

Use strtoull(3) instead, which allows error checking. Check that the
entire string has been parsed correctly without overflows/underflows,
but not that the __u64 (the type of struct landlock_net_port_attr.port)
is a valid __u16 port: that is already done by the kernel.

Fixes: 5e990dcef12e ("samples/landlock: Support TCP restrictions")
Signed-off-by: Matthieu Buffet <matthieu@buffet.re>
Link: https://lore.kernel.org/r/20241019151534.1400605-2-matthieu@buffet.re
Signed-off-by: Mickaël Salaün <mic@digikod.net>