btrfs: send: directly return strcmp() result when comparing recorded refs

There's no point in converting the return values from strcmp() as all we
need is that it returns a negative value if the first argument is less
than the second, a positive value if it's greater and 0 if equal. We do
not have a need for -1 instead of any other negative value and no need
for 1 instead of any other positive value - that's all that rb_find()
needs and no where else we need specific negative and positive values.

So remove the intermediate local variable and checks and return directly
the result from strcmp().

This also reduces the module's text size.

Before:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1888116	 161347	  16136	2065599	 1f84bf	fs/btrfs/btrfs.ko

After:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1888052	 161347	  16136	2065535	 1f847f	fs/btrfs/btrfs.ko

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>