mempool: factor out a mempool_alloc_from_pool helper

mempool: factor out a mempool_alloc_from_pool helper

Add a helper for the mempool_alloc slowpath to better separate it from the
fast path, and also use it to implement mempool_alloc_preallocated which
shares the same logic.

[hughd@google.com: fix lack of retrying with __GFP_DIRECT_RECLAIM]
[vbabka@suse.cz: really use limited flags for first mempool attempt]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20251113084022.1255121-7-hch@lst.de
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>