powerpc: Make sure that include/asm-powerpc/spinlock.h does not trigger compilation warnings

When compiling kernel modules for ppc that include <linux/spinlock.h>,
gcc prints a warning message every time it encounters a function
declaration where the inline keyword appears after the return type.
This makes sure that the order of the inline keyword and the return
type is as gcc expects it.  Additionally, the __inline__ keyword is
replaced by inline, as checkpatch expects.

Signed-off-by: Bart Van Assche <bart.vanassche@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>