ANDROID: mm/memfd-ashmem-shim: Simplify buffer name retrieval

The current way of getting the name for a buffer always requires a
buffer to be allocated for the name to be copied into. This is
inefficient, as names for shmem buffers are always stored in the
same field, and they do not change.

Therefore, simplify the name retrieval to just read the buffer name
from the field it is always stored in for shmem buffers. This also
aligns the code to what is present on the android16-6.12 branch.

Bug: 401214613
Bug: 111903542
Change-Id: Idd7b2d16601c890b78bd5705c92842bee470e75c
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>