irqchip/riscv-imsic: Separate next and previous pointers in IMSIC vector

Currently, there is only one "move" pointer in struct imsic_vector so
during vector movement the old vector points to the new vector and new
vector points to itself.

To support forced cleanup of the old vector, add separate "move_next" and
"move_prev" pointers to struct imsic_vector, where during vector movement
the "move_next" pointer of the old vector points to the new vector and the
"move_prev" pointer of the new vector points to the old vector.

Both "move_next" and "move_prev" pointers are cleared separately by
__imsic_local_sync() with a restriction that "move_prev" on the new
CPU is cleared only after the old CPU has cleared "move_next".

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250217085657.789309-8-apatel@ventanamicro.com