mdadm: util.c fix coverity issues

mdadm: util.c fix coverity issues

Fixing the following coding errors the coverity tools found:

* Event check_return: Calling "open" without checking return value
* Event check_return: Calling "lseek(fd, sector_size, 0)" without
checking return value.
* Event leaked_handle: Handle variable "fd" going out of scope leaks
the handle.
* Event leaked_storage: Variable "dir" going out of scope leaks the
storage it points to.
* Event fixed_size_dest: You might overrun the 32-character fixed-size
string "st->devnm" by copying "_devnm" without checking the length.
* Event fixed_size_dest: You might overrun the 32-character fixed-size
string "container" by copying "dev" without checking the length.

Signed-off-by: Nigel Croxon <ncroxon@redhat.com>