MEDIUM: buffer: use b_alloc() to allocate and initialize a buffer

MEDIUM: buffer: use b_alloc() to allocate and initialize a buffer

b_alloc() now allocates a buffer and initializes it to the size specified
in the pool minus the size of the struct buffer itself. This ensures that
callers do not need to care about buffer details anymore. Also this never
applies memory poisonning, which is slow and useless on buffers.