disk: efi: correct the allocation size for mbr header in stack

use ALLOC_CACHE_ALIGN_BUFFER_PAD for mbr header allocation
in stack to fix alloc issue in is_gpt_valid()

this patch fix also issue for GPT partition handling
with blocksize != 512 in set_protective_mbr()

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>