btrfs: sysfs: handle value associated with read balancing policy

btrfs: sysfs: handle value associated with read balancing policy

Enable specifying additional configuration values along the RAID1
balancing read policy in a single input string.

Update btrfs_read_policy_to_enum() to parse and handle a value
associated with the policy in the format "policy:value", the value part
if present is converted to 64-bit integer.

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