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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.

Cc: stable@kernel.org
Found-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>