Fix undefined reference on spl_mutex_spin_max().

Commit 3160d4f56bf35492e9c400094f8c1ff2066d4459 changed the set of
conditions under which spl_mutex_spin_max would be implemented as a
function by changing an #if in sys/mutex.h. The corresponding
implementation file spl-mutex.c, however, has not been updated to
reflect the change. This results in undefined reference errors on
spl_mutex_spin_max under the following condition:

((!CONFIG_SMP || CONFIG_DEBUG_MUTEXES) && HAVE_MUTEX_OWNER && HAVE_TASK_CURR)

This patch fixes the issue by using the same #if in sys/mutex.h and
spl-mutex.c.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue zfsonlinux/zfs#851