sync: Fix error paths

Check the return value of get_unused_fd to make sure a valid
file descriptor is returned.

Make sure to call put_unused_fd even if an error occurs before
the fd can be used.

Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
Change-Id: I0fe8f78d9e30ecfc4d271c5d875424543dae2d0f