net: buf: add max allocation size to allocation info

Previously, there was no way to determine maximum number of bytes
that can be allocated using only net_buf structure. This commit
introduces such field.

Moreover, this commit fixes an issue where allocation of less than
maximum number of bytes from a fixed buffer pool would set buffer's
size to this number instead of the whole buffer size.

Signed-off-by: Konrad Derda <konrad.derda@nordicsemi.no>