btrfs: send: avoid extra calls to strlen() in gen_unique_name()

Since 'snprintf()' returns the number of characters which would
be emitted and output truncation is handled by 'ASSERT()', it
should be safe to use that return value instead of the subsequent
calls to 'strlen()' in 'gen_unique_name()'.

This also reduces the module's text size.

Before:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1897006	 161571	  16136	2074713	 1fa859	fs/btrfs/btrfs.ko

After:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1896848	 161571	  16136	2074555	 1fa7bb	fs/btrfs/btrfs.ko

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Reviewed-by: Filipe Manana <fdmanana@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>