lockref: include mutex.h rather than reinvent arch_mutex_cpu_relax

arch_mutex_cpu_relax is already conditionally defined in mutex.h, so
simply include that header rather than replicate the code here.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>