btrfs: sysfs: add btrfs_read_policy_to_enum() helper and refactor read policy store
Introduce btrfs_read_policy_to_enum() helper to simplify the conversion
of a string read policy to its corresponding enum value. This reduces
duplication and improves code clarity in btrfs_read_policy_store().
The parameter is copied locally to allow modification, enabling the
separation of the method and its value. This prepares for the addition of
more functionality in subsequent patches.
Signed-off-by: Anand Jain <anand.jain@oracle.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>