include/asm-generic/atomic-long.h: force inlining of some atomic_long operations

Sometimes gcc mysteriously doesn't inline
very small functions we expect to be inlined. See

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66122

With this .config:
http://busybox.net/~vda/kernel_config_OPTIMIZE_INLINING_and_Os,
atomic_long_inc(), atomic_long_dec() and atomic_long_add()
functions get deinlined about 40 times. Examples of disassembly:

<atomic_long_inc> (21 copies, 147 calls):
       55                      push   %rbp
       48 89 e5                mov    %rsp,%rbp
       f0 48 ff 07             lock incq (%rdi)
       5d                      pop    %rbp
       c3                      retq

<atomic_long_dec> (4 copies, 14 calls) is similar to inc.

<atomic_long_add> (11 copies, 41 calls):
       55                      push   %rbp
       48 89 e5                mov    %rsp,%rbp
       f0 48 01 3e             lock add %rdi,(%rsi)
       5d                      pop    %rbp
       c3                      retq

This patch fixes this via s/inline/__always_inline/.
Code size decrease after the patch is ~1.3k:

    text     data      bss       dec     hex filename
92203657 20826112 36417536 149447305 8e86289 vmlinux
92202377 20826112 36417536 149446025 8e85d89 vmlinux4_atomiclong_after

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Graf <tgraf@suug.ch>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>