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 <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Chris Zankel <[e-mail]>

include/asm-xtensa/rwsem.h

···
 #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>