FROMLIST: BACKPORT: maple_tree: Refine mas_preallocate() node calculations

Calculate the number of nodes based on the pending write action instead
of assuming the worst case.

This addresses a performance regression introduced in platforms that
have longer allocation timing.

Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>

Link: https://lore.kernel.org/lkml/20230601021605.2823123-14-Liam.Howlett@oracle.com/


[surenb: adjust node_size calculation, allow to store a slot when
possible]

Bug: 274059236
Change-Id: I1db402fb463ee1e391081d2d81c34619f15713ac
Signed-off-by: Suren Baghdasaryan <surenb@google.com>