Fix decoding of fcntl/fcntl64 operation argument

Fix decoding of fcntl/fcntl64 operation argument

Consistently treat operation argument of fcntl/fcntl64 syscalls as int
to match the kernel behaviour.

* fcntl.c (print_fcntl, SYS_FUNC(fcntl), SYS_FUNC(fcntl64)): Assign
2nd argument of syscall to a variable of type unsigned int
and use it in all subsequent checks and lookups.
* tests/struct_flock.c (invoke_test_syscall): New function.
(test_flock_einval, test_flock): Use it.
* tests/fcntl.c (test_flock64_einval): Use it.
* tests/fcntl64.c (test_flock64_einval, test_flock64): Use it.