Revert "zhack: Add 'feature disable' command"

This reverts commit 8302528617 and
ebecfcd699 which broke the build.
While these patches do apply cleanly and passed previous test
runs they need to be updated to account for the changes made in
commit 241b541574.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #3878

lib/libzpool/kernel.c

@@ -647,6 +647,8 @@ vn_open(char *path, int x1, int flags, int mode, vnode_t **vpp, int x2, int x3)
 #ifdef __linux__
 		flags |= O_DIRECT;
 #endif
+		/* We shouldn't be writing to block devices in userspace */
+		VERIFY(!(flags & FWRITE));
 	}
 
 	if (flags & FCREAT)