btrfs: send: remove duplicated logic from fs_path_reset()

There's duplicated logic in both branches of the if statement, so move it
outside the branches.

This also reduces the object code size.

Before this change:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1746279	 163600	  16920	1926799	 1d668f	fs/btrfs/btrfs.ko

After this change:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1746047	 163592	  16920	1926559	 1d659f	fs/btrfs/btrfs.ko

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>