lib/extable.c : Removes an expensive integer divide in search_extable()

Actual code let compiler generates idiv instruction on x86.

Using a right shift is OK here and readable as well.

Before patch

  10:   57                      push   %edi
  11:   56                      push   %esi
  12:   89 d6                   mov    %edx,%esi
  14:   53                      push   %ebx
  15:   89 c3                   mov    %eax,%ebx
  17:   eb 22                   jmp    3b <search_extable+0x2b>
  19:   89 f0                   mov    %esi,%eax
  1b:   ba 02 00 00 00          mov    $0x2,%edx
  20:   29 d8                   sub    %ebx,%eax
  22:   89 d7                   mov    %edx,%edi
  24:   c1 f8 03                sar    $0x3,%eax
  27:   99                      cltd
  28:   f7 ff                   idiv   %edi
  2a:   8d 04 c3                lea    (%ebx,%eax,8),%eax
  2d:   39 08                   cmp    %ecx,(%eax)

...

After patch

00000010 <search_extable>:

  10:   53                      push   %ebx
  11:   89 c3                   mov    %eax,%ebx
  13:   eb 18                   jmp    2d <search_extable+0x1d>
  15:   89 d0                   mov    %edx,%eax
  17:   29 d8                   sub    %ebx,%eax
  19:   c1 f8 04                sar    $0x4,%eax
  1c:   8d 04 c3                lea    (%ebx,%eax,8),%eax
  1f:   39 08                   cmp    %ecx,(%eax)

...

index 463f456..179c087 100644
@@ -57,10 +57,10 @@ search_extable(const struct exception_table_entry *first,
 	while (first <= last) {
 		const struct exception_table_entry *mid;
 
-		mid = (last - first) / 2 + first;
+		mid = ((last - first) >> 1) + first;
 		/*
-		 * careful, the distance between entries can be
-		 * larger than 2GB:
+		 * careful, the distance between value and insn
+		 * can be larger than MAX_LONG:
 		 */
 		if (mid->insn < value)
 			first = mid + 1;