btrfs-progs: per-thread, per-call pretty buffer

btrfs-progs: per-thread, per-call pretty buffer

We don't need callers to manage string storage for each pretty_sizes()
call.  We can use a macro to have per-thread and per-call static storage
so that pretty_sizes() can be used as many times as needed in printf()
arguments without requiring a bunch of supporting variables.

This lets us have a natural interface at the cost of requiring __thread
and TLS from gcc and a small amount of static storage.  This seems
better than the current code or doing something with illegible format
specifier macros.

Signed-off-by: Zach Brown <zab@redhat.com>
Acked-by: Wang Shilong <wangs.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>