lib/extable.c: remove 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)
...

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>