maria_repair_parallel() clears the MY_THREAD_SPECIFIC flag for allocations
since it uses different threads. But it still did one _ma_alloc_buffer()
call as thread-specific which would later assert if another thread needed
to extend the buffer with realloc.

This patch, due to Monty, removes the MY_THREAD_SPECIFIC flag for
allocations that need to realloc in different threads, and preserves
it for those that are allocated/freed in the user's thread.

Also fixes MDEV-33562.
Reviewed-by: Monty <monty@mariadb.org>
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>