md: fix error path when duplicate name is found on md device creation.

md: fix error path when duplicate name is found on md device creation.

commit 1ec22eb2b4a2e1a763106bce36b11c02eaa84e61 upstream.

When an md device is created by name (rather than number) we need to
check that the name is not already in use.  If this check finds a
duplicate, we return an error without dropping the lock or freeing
the newly create mddev.
This patch fixes that.

Found-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>