ANDROID: mm/memfd-ashmem-shim: Fix variable length array usage

The size of the buffer used to retrieve the memfd file name is
currently calculated at runtime, making the buffer a variable
length array. However, all of the terms used in the buffer size
calculation are known at compile time, so use compile time constants
for the calculation.

Bug: 399839316
Change-Id: Ie1edf9a28f735ebeffab07f64efc4de45f1f095a
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>