maple_tree: do not preallocate nodes for slot stores

mas_preallocate() defaults to requesting 1 node for preallocation and then
,depending on the type of store, will update the request variable.  There
isn't a check for a slot store type, so slot stores are preallocating the
default 1 node.  Slot stores do not require any additional nodes, so add a
check for the slot store case that will bypass node_count_gfp().  Update
the tests to reflect that slot stores do not require allocations.

User visible effects of this bug include increased memory usage from the
unneeded node that was allocated.

Link: https://lkml.kernel.org/r/20231213205058.386589-1-sidhartha.kumar@oracle.com
Fixes: 0b8bb544

 ("maple_tree: update mas_preallocate() testing")
Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Peng Zhang <zhangpeng.00@bytedance.com>
Cc: <stable@vger.kernel.org>	[6.6+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>