sys/libkern.h: Add type conversion helpers

Add helper functions for 32 and 64 bit unsigned to signed integers
conversions.

Reviewed by:
Sponsored by:		Alstom
Obtained from:		Semihalf
Differential revision:	https://reviews.freebsd.org/D33162

sys/sys/libkern.h

@@ -226,6 +226,22 @@ rindex(const char *p, int ch)
 	return (strrchr(p, ch));
 }
 
+static __inline int64_t
+signed_extend64(uint64_t bitmap, int lsb, int width)
+{
+
+	return ((int64_t)(bitmap << (63 - lsb - (width - 1)))) >>
+	    (63 - (width - 1));
+}
+
+static __inline int32_t
+signed_extend32(uint32_t bitmap, int lsb, int width)
+{
+
+	return ((int32_t)(bitmap << (31 - lsb - (width - 1)))) >>
+	    (31 - (width - 1));
+}
+
 /* fnmatch() return values. */
 #define	FNM_NOMATCH	1	/* Match failed. */