tests/fcntl-common.c: define and initialize errstr

tests/fcntl-common.c: define and initialize errstr

Initialize errstr inside invoke_test_syscall so that its users
won't have to go into trouble of caring about clobbering errno.

* tests/fcntl-common.c (errstr): New variable.
(invoke_test_syscall): Initialize it.
(test_flock_einval, test_flock64_einval, test_flock,
test_f_owner_ex_type_pid): Use it.
* tests/fcntl.c (test_flock64_undecoded): Likewise.
* tests/fcntl64.c (test_flock64_lk64): Likewise.