part: Allocate only one legacy_mbr buffer

part: Allocate only one legacy_mbr buffer

Commit ff98cb90514d ("part: extract MBR signature from partitions")
blindly switched allocated by ALLOC_CACHE_ALIGN_BUFFER buffer type from
"unsigned char" to "legacy_mbr" which caused allocation of size =
(typeof(legacy_mbr) * dev_desc->blksize) instead of just space enough
for "legacy_mbr" structure.

Signed-off-by: Alexey Brodkin <[email protected]>
Cc: Rob Clark <[email protected]>
Cc: Peter Jones <[email protected]>
Cc: Alexander Graf <[email protected]>
Cc: Tom Rini <[email protected]>