The .write/.read file operations callbacks can be retired since
support for .read_iter/.write_iter and .aio_read/.aio_write has
been added. The vfs_write()/vfs_read() entry functions will
select the correct interface for the kernel. This is desirable
because all VFS write/read operations now rely on common code.
This change also add the generic write checks to make sure that
ulimits are enforced correctly on write.