mutex: __used is needed for function referenced only from inline asm

Impact: fix build failure on llvm-gcc-4.2

According to the gcc manual, the 'used' attribute should be applied to
functions referenced only from inline assembly.
This fixes a build failure with llvm-gcc-4.2, which deleted
__mutex_lock_slowpath, __mutex_unlock_slowpath.

Signed-off-by: Török Edwin <edwintorok@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>