Btrfs-progs: fix wrong manpage of defrag command

'btrfs filesystem defrag' has an option '-t', whose manpage says

"Any extent bigger than threshold given by -t option, will be
considered already defragged. Use 0 to take the kernel default, and
use 1 to say every single extent must be rewritten."

Here 'use 0' still works, it refers to the default value(256K), however,
'use 1' is an obvious typo, it should be -1, which means the largest value
it can be.

Right now, we use parse_size() which no more allow value '-1', so in
order to keep the manpage correct, this updates it to only keep value '0'.

If you want to make sure every single extent is rewritten, please use a fairly
large size, say 1G.

Reported-by: Sebastian Ochmann <ochmann@informatik.uni-bonn.de>
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.cz>

Documentation/btrfs-filesystem.txt

@@ -41,8 +41,7 @@ The start position and the number of bytes to defragment can be specified by
 start and len using '-s' and '-l' options below.
 Any extent bigger than threshold given by '-t' option, will be considered
 already defragged.
-Use 0 to take the kernel default, and use 1 to
-say every single extent must be rewritten.
+Use 0 to take the kernel default.
 You can also turn on compression in defragment operations.
 +
 `Options`