XTENSA: warn about including <asm/rwsem.h> directly.

For consistency with other architectures, warn about including
<asm-xtensa/rwsem.h> directly.
Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Chris Zankel <chris@zankel.net>

include/asm-xtensa/rwsem.h

@@ -13,6 +13,10 @@
 #ifndef _XTENSA_RWSEM_H
 #define _XTENSA_RWSEM_H
 
+#ifndef _LINUX_RWSEM_H
+#error "Please don't include <asm/rwsem.h> directly, use <linux/rwsem.h> instead."
+#endif
+
 #include <linux/list.h>
 #include <linux/spinlock.h>
 #include <asm/atomic.h>