test/test_syscalls.c: allow EBADF in fcheck_stat() (#631)

test/test_syscalls.c: allow EBADF in fcheck_stat() (#631)

Test test/test_examples.py::test_passthrough_hp[False] fails because, on
kernels >= 5.14, fstat() will return -EBADF:

3 [check_unlinked_testfile] fcheck_stat() - fstat: Bad file descriptor
4 [check_unlinked_testfile] fcheck_stat() - fstat: Bad file descriptor
5 [check_unlinked_testfile] fcheck_stat() - fstat: Bad file descriptor
9 [check_unlinked_testfile] fcheck_stat() - fstat: Bad file descriptor
...

This patch simply whitelists the EBADF errno code.

Signed-off-by: Luís Henriques <lhenriques@suse.de>
Co-authored-by: Luís Henriques <lhenriques@suse.de>