Btrfs-progs: new helper to parse string to u64 for btrfs

Btrfs-progs: new helper to parse string to u64 for btrfs

There are many places that need parse string to u64 for btrfs commands,
in fact, we do such things *too casually*, using atoi/atol/atoll..is not
right at all, and even we don't check whether it is a valid string.

Let's do everything more gracefully, we introduce a new helper
arg_strtou64() which will do all the necessary checks.If we fail to
parse string to u64, we will output message and exit directly, this is
something like what usage() is doing. It is ok to not return erro to
it's caller, because this function should be called when parsing arg
(just like usage!)

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>