FIX: Verify if array name doesn't exist already

FIX: Verify if array name doesn't exist already

When e.g. array name (an) is correct and it is the same as container name (cn),
file element creation /dev/md/an will replace /dev/md/cn.
This can cause that user cannot access container using /dev/md/cn.

Verify during array creation if chosen name is not already existing
one.

[Changed to use map_by_name() rather than stat() to determine prior
 existence - NeilBrown]

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>