lib/test_vmalloc.c: replace RWSEM to SRCU for setup

The test has the initialization step during which threads are created.  To
prevent the workers from starting prematurely a write lock was previously
used by the main setup thread, while each worker would block on a read
lock.

Replace this RWSEM based synchronization with a simpler SRCU based
approach.  Which does two basic steps:

- Main thread wraps the setup phase in an SRCU read-side critical
  section.  Pair of srcu_read_lock()/srcu_read_unlock().
- Each worker calls synchronize_srcu() on entry, ensuring it waits for
  the initialization phase to be completed.

This patch eliminates the need for down_read()/up_read() and
down_write()/up_write() pairs thus simplifying the logic and improving
clarity.

[urezki@gmail.com: fix compile error with CONFIG_TINY_RCU]
  Link: https://lkml.kernel.org/r/20250420142029.103169-1-urezki@gmail.com
Link: https://lkml.kernel.org/r/20250417161216.88318-1-urezki@gmail.com


Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Adrian Huang <ahuang12@lenovo.com>
Tested-by: Adrian Huang <ahuang12@lenovo.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Christop Hellwig <hch@infradead.org>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>