bus: mhi: host: Use kzalloc_flex

bus: mhi: host: Use kzalloc_flex

Change kzalloc + kzalloc to just kzalloc with a flexible array member.

Add __counted_by for extra runtime analysis when requested.

Move counting assignment immediately after allocation as required by
__counted_by.

Move mhi_buf definition as a complete definition as needed for flex
arrays. It's not a pointer anymore.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
[mani: squashed https://lore.kernel.org/mhi/20260317-mhi-invalid-free-mhi-buffers-v1-1-8418a3ad604f@oss.qualcomm.com]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Link: https://patch.msgid.link/20260312045921.7663-1-rosenp@gmail.com