XArray: Add xa_limit_16b

A 16-bit limit is a more common limit than I had realised.  Make it
generally available.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

include/linux/xarray.h

@@ -229,9 +229,10 @@ static inline int xa_err(void *entry)
  *
  * This structure is used either directly or via the XA_LIMIT() macro
  * to communicate the range of IDs that are valid for allocation.
- * Two common ranges are predefined for you:
+ * Three common ranges are predefined for you:
  * * xa_limit_32b	- [0 - UINT_MAX]
  * * xa_limit_31b	- [0 - INT_MAX]
+ * * xa_limit_16b	- [0 - USHRT_MAX]
  */
 struct xa_limit {
 	u32 max;
@@ -242,6 +243,7 @@ struct xa_limit {
 
 #define xa_limit_32b	XA_LIMIT(0, UINT_MAX)
 #define xa_limit_31b	XA_LIMIT(0, INT_MAX)
+#define xa_limit_16b	XA_LIMIT(0, USHRT_MAX)
 
 typedef unsigned __bitwise xa_mark_t;
 #define XA_MARK_0		((__force xa_mark_t)0U)