btrfs-progs: make the device scan logic more clear

btrfs-progs: make the device scan logic more clear

1. Use long option to replace the original strcmp() to parse
the "--all-devices".
2. the "int ret" is defined in 2 places, just define it once
and make the return pattern into "goto + single return".

This does not change the actual scan procedure and return values.
Just make it clear, the original seems a little confusing.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>