btrfs: create helper function __check_raid_min_devices()

btrfs: create helper function __check_raid_min_devices()

move a section of btrfs_rm_device() code to check for min number of the
devices into the function __check_raid_min_devices()

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Orabug: 26287586

(Cherry picked from commit f1fa7f264250f2cb60119aca3fd114c3f47070c2,
 drop the second argument of the btrfs_dev_replace_lock/unlock to
 match with the current kernel api)

Signed-off-by: Shan Hai <shan.hai@oracle.com>
Reviewed-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>