asm-generic: force inlining of get_order() to work around gcc10 poor decision

When building mpc885_ads_defconfig with gcc 10.1,
the function get_order() appears 50 times in vmlinux:

  [linux]# ppc-linux-objdump -x vmlinux | grep get_order | wc -l
  50

  [linux]# size vmlinux
     text	   data	    bss	    dec	    hex	filename
  3842620	 675624	 135160	4653404	 47015c	vmlinux

In the old days, marking a function 'static inline' was forcing GCC to
inline, but since commit ac7c3e4ff4 ("compiler: enable
CONFIG_OPTIMIZE_INLINING forcibly") GCC may decide to not inline a
function.

It looks like GCC 10 is taking poor decisions on this.

get_order() compiles into the following tiny function, occupying 20
bytes of text.

  0000007c <get_order>:
    7c:   38 63 ff ff     addi    r3,r3,-1
    80:   54 63 a3 3e     rlwinm  r3,r3,20,12,31
    84:   7c 63 00 34     cntlzw  r3,r3
    88:   20 63 00 20     subfic  r3,r3,32
    8c:   4e 80 00 20     blr

By forcing get_order() to be __always_inline, the size of text is
reduced by 1940 bytes, that is almost twice the space occupied by
50 times get_order()

  [linux-powerpc]# size vmlinux
     text	   data	    bss	    dec	    hex	filename
  3840680	 675588	 135176	4651444	 46f9b4	vmlinux

Link: https://lkml.kernel.org/r/96c6172d619c51acc5c1c4884b80785c59af4102.1602949927.git.christophe.leroy@csgroup.eu
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/getorder.h

@@ -26,7 +26,7 @@
  *
  * The result is undefined if the size is 0.
  */
-static inline __attribute_const__ int get_order(unsigned long size)
+static __always_inline __attribute_const__ int get_order(unsigned long size)
 {
 	if (__builtin_constant_p(size)) {
 		if (!size)