libcutils: ashmem: Avoid doing fd checks for ashmem calls

Callers already verify that they are calling ashmem API on a valid fd by
calling ashmem_valid first. Lets make the fstat syscall only if the
ioctl returns -ENOTTY. This means in the regular case, only 1 syscall is
needed (ioctl) vs the current 2 (fstat+ioctl).

Some data to show improvements in reduction of vfs_getattr calls in the
kernel by 10x when doing a camera.

Test: Boot and camera CTS
Bug: 111418894
Change-Id: I992620bbe44355e54ba19eeac81da586c5e5a6e0
Signed-off-by: Joel Fernandes <joelaf@google.com>