shared/machine-pool: fix another mkfs.btrfs checking

shared/machine-pool: fix another mkfs.btrfs checking

Fixes:
       Message: Process 806 (systemd-importd) of user 0 dumped core.

                Stack trace of thread 806:
                #0  0x00007f5eaeff7227 raise (libc.so.6)
                #1  0x00007f5eaeff8e8a abort (libc.so.6)
                #2  0x000055b6d3418f4f log_assert_failed (systemd-importd)
                #3  0x000055b6d3409daf safe_close (systemd-importd)
                #4  0x000055b6d33c25ea closep (systemd-importd)
                #5  0x000055b6d33c38d9 setup_machine_directory (systemd-importd)
                #6  0x000055b6d33b8536 method_pull_tar_or_raw (systemd-importd)
                #7  0x000055b6d33ed097 method_callbacks_run (systemd-importd)
                #8  0x000055b6d33ef929 object_find_and_run (systemd-importd)
                #9  0x000055b6d33eff6b bus_process_object (systemd-importd)
                #10 0x000055b6d3447f77 process_message (systemd-importd)
                #11 0x000055b6d344815a process_running (systemd-importd)
                #12 0x000055b6d3448a10 bus_process_internal (systemd-importd)
                #13 0x000055b6d3448ae1 sd_bus_process (systemd-importd)
                #14 0x000055b6d3449779 time_callback (systemd-importd)
                #15 0x000055b6d3454ff4 source_dispatch (systemd-importd)
                #16 0x000055b6d34562b9 sd_event_dispatch (systemd-importd)
                #17 0x000055b6d34566f8 sd_event_run (systemd-importd)
                #18 0x000055b6d33ba72a bus_event_loop_with_idle (systemd-importd)
                #19 0x000055b6d33b95bc manager_run (systemd-importd)
                #20 0x000055b6d33b9766 main (systemd-importd)
                #21 0x00007f5eaefe2a00 __libc_start_main (libc.so.6)
                #22 0x000055b6d33b5569 _start (systemd-importd)