Retire .write/.read file operations

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.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #5587 
Closes #5673

config/kernel-vfs-rw-iterate.m4

@@ -25,3 +25,24 @@ AC_DEFUN([ZFS_AC_KERNEL_VFS_RW_ITERATE],
 		AC_MSG_RESULT(no)
 	])
 ])
+
+dnl #
+dnl # Linux 4.1.x API
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_GENERIC_WRITE_CHECKS],
+	[AC_MSG_CHECKING([whether generic_write_checks() takes kiocb])
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/fs.h>
+
+	],[
+		struct kiocb *iocb = NULL;
+		struct iov_iter *iov = NULL;
+		generic_write_checks(iocb, iov);
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_GENERIC_WRITE_CHECKS_KIOCB, 1,
+			[generic_write_checks() takes kiocb])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -104,6 +104,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_LSEEK_EXECUTE
 	ZFS_AC_KERNEL_VFS_ITERATE
 	ZFS_AC_KERNEL_VFS_RW_ITERATE
+	ZFS_AC_KERNEL_GENERIC_WRITE_CHECKS
 	ZFS_AC_KERNEL_KMAP_ATOMIC_ARGS
 	ZFS_AC_KERNEL_FOLLOW_DOWN_ONE
 	ZFS_AC_KERNEL_MAKE_REQUEST_FN