+ * Extend a 32 bit TSF to nearest 64 bit TSF value.
+ * When the adapter is a STATION, its local TSF is periodically modified by
+ * the hardware to match the BSS TSF (as received in beacon packets), and
+ * rstamp may appear to be from the future or from the past (with reference
+ * to the current local TSF) because of jitter. This is mostly noticable in
+ * highly congested channels. The code uses signed modulo arithmetic to
+ * handle both past/future cases and signed-extension to avoid branches.
+ * Test cases:
+ * extend(0x0000001200000004, 0x00000006) == 0x0000001200000006
+ * extend(0x0000001200000004, 0x00000002) == 0x0000001200000002
+ * extend(0x0000001200000004, 0xfffffffe) == 0x00000011fffffffe ! tsfhigh--
+ * extend(0x000000127ffffffe, 0x80000002) == 0x0000001280000002
+ * extend(0x0000001280000002, 0x7ffffffe) == 0x000000127ffffffe
+ * extend(0x00000012fffffffc, 0xfffffffe) == 0x00000012fffffffe
+ * extend(0x00000012fffffffc, 0xfffffffa) == 0x00000012fffffffa
+ * extend(0x00000012fffffffc, 0x00000002) == 0x0000001300000002 ! tsfhigh++