fs_mgr: properly propagate exec child errors

Child status was being requested, but ignored, so if the exec failed
with something like file not found or permissions denied the return
value with be 0 (success). Passing in NULL instead of &status causes
the failure to be properly returned from execvp instead of in status.

Test: erase f2fs userdata on device without /system/bin/make_f2fs and
      verify device boots into recovery to format the partition instead
      of believing the format succeeded and going into infinite f2fs.fsck
      loop.
Bug: 62901965
Signed-off-by: Ben Fennema <fennema@google.com>
Change-Id: Ia5bbf09d5a666402cba8437abcc56775583ba6d2