btrfs-progs: Improve the parse_size() error message
When using parse_size(), even non-numeric value is passed, it will only
give error message "ERROR: size value is empty", which is quite
confusing for end users.
This patch will introduce more meaningful error message for the
following new cases
1) Invalid size string (non-numeric string)
2) Minus size value (like "-1K")
Also this patch will take full use of endptr returned by strtoll() to
reduce unneeded loop.
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>