`resize`: warn if resizing below 256m

[kdave]

https://bugzilla.kernel.org/show_bug.cgi?id=118111 continued.

The lower limit for resizing is 256m and checked by kernel, but the error message is generic when printed from btrfs fi resize:

$ btrfs fi resize 120M mnt
Resize device id 1 (/dev/loop1) from 2.00GiB to 120.00MiB
ERROR: unable to resize 'mnt': Invalid argument

This needs to parse the resize string and check device size agains the absolute or diff size.