btrfs: simplify transid initialization in btrfs_ioctl_wait_sync

A small code simplification, move the default value of transid to its
initialization and remove the else-statement.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>