btrfs-progs: Introduce get_unit_mode_from_arg for common use

btrfs-progs: Introduce get_unit_mode_from_arg for common use

We are using separate code for parsing unit mode in current code,
better to use common function.

This patch introduces a common function to specify units as arguments
and a common help message, to make every tool in btrfs having same unit
argument.

The benefits are:
1: Unify current tool's arguments for unit
2: Make tools in future easy to implement such argument
3: Changes (enhancement) in common function have effect on all
   relative tools

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>