Fix/improve the btrfs_resize function

[vpodzime]

In order to resize a btrfs file system one needs to specify sizes of space that should be used on devices used by the btrfs file system. Simply calling btrfs filesystem resize SIZE MNT_POINT only resizes the first device not the whole file system. btrfs filesystem resize DEV_ID:SIZE MNT_POINT resizes a particular device.

Thus we need to add a parameter to bd_btrfs_resize() that will allow callers to specify a device and if none is specified, we need to calculate how much space we need to take from devices used by the file system to get the given total size (as one would expect from such function).