samples: fix pidfd-metadata compilation

Define __NR_pidfd_send_signal if it isn't to prevent a compilation error.

To make pidfd-metadata compile on all arches, irrespective of whether
or not syscall numbers are assigned, define the syscall number to -1.
If it isn't defined this will cause the kernel to return -ENOSYS.

Fixes: 43c6afee48d4 ("samples: show race-free pidfd metadata access")
Reported-by: Arnd Bergmann <>
Reported-by: Guenter Roeck <>
Cc: Christian Brauner <>
Signed-off-by: Guenter Roeck <>
[ tweak commit message]
Signed-off-by: Christian Brauner <>
1 file changed