Add rwsem_tryupgrade for 4.9.20-rt16 kernel (SPL)

Add rwsem_tryupgrade for 4.9.20-rt16 kernel (SPL)

(This is the SPL backported code from 11d0525cb)

The RT rwsem implementation was changed to allow multiple readers
as of the 4.9.20-rt16 patch set.  This results in a build failure
because the existing implementation was forced to directly access
the rwsem structure which has changed.

While this could be accommodated by adding additional compatibility
code.  This patch resolves the build issue by simply assuming the
rwsem can never be upgraded.  This functionality is a performance
optimization and all callers must already handle this case.

Converting the last remaining use of __SPIN_LOCK_UNLOCKED to
spin_lock_init() was additionally required to get a clean build.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7589