mdadm: Monitor.c fix coverity issues

mdadm: Monitor.c fix coverity issues

Fixing the following coding errors the coverity tools found:

* Event check_return: Calling "fcntl(fd, 2, 1)" without checking
return value. This library function may fail and return an error code.

* Dereferencing "sl", which is known to be "NULL".

* Event fixed_size_dest: You might overrun the 32-character fixed-size
string "devnm" by copying "tmp" without checking the length.

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