Mutex ASSERT on self deadlock

Mutex ASSERT on self deadlock

Generate an assertion if we're going to deadlock the system by
attempting to acquire a mutex the process is already holding.

There are currently no known instances of this under normal
operation, but it _might_ be possible when using a ZVOL as a
swap device.  I want to ensure we catch this immediately if it
were to occur.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>