mm: zpool: remove zpool_malloc_support_movable()

zpool_malloc_support_movable() always returns true for zsmalloc, the only
remaining zpool driver.  Remove it and set the gfp flags in
zswap_compress() accordingly.  Opportunistically use GFP_NOWAIT instead of
__GFP_NOWARN | __GFP_KSWAPD_RECLAIM for conciseness as they are
equivalent.

Link: https://lkml.kernel.org/r/20250305061134.4105762-6-yosry.ahmed@linux.dev


Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>