This patch carries out a refactoring of the sendpage functionality in
the write_data() function:
1. Reorganize the logic used to select the sock_sendpage function.
2. Streamline the data sending loop by reducing conditional branches and
eliminating labels.
3. Adjust the error handling for -EINTR and -EAGAIN to make the code
cleaner and easier to follow.
This patch doesn't change any functionality.