MdeModulePkg/Variable/RuntimeDxe: introduce PcdMaxVolatileVariableSize

MdeModulePkg/Variable/RuntimeDxe: introduce PcdMaxVolatileVariableSize

The variable driver doesn't distinguish "non-volatile non-authenticated"
variables from "volatile non-authenticated" variables, when checking
individual variable sizes against the permitted maximum.
PcdMaxVariableSize covers both kinds.

This prevents volatile non-authenticated variables from carrying large
data between UEFI drivers, despite having no flash impact. One example is
EFI_TLS_CA_CERTIFICATE_VARIABLE, which platforms might want to create as
volatile on every boot: the certificate list can be several hundred KB in
size.

Introduce PcdMaxVolatileVariableSize to represent the limit on individual
volatile non-authenticated variables. The default value is zero, which
makes Variable/RuntimeDxe fall back to PcdMaxVariableSize (i.e. the
current behavior). This is similar to the PcdMaxAuthVariableSize fallback.

Whenever the size limit is enforced, consult MaxVolatileVariableSize as
the last option, after checking
- MaxAuthVariableSize for VARIABLE_ATTRIBUTE_AT_AW,
- and MaxVariableSize for EFI_VARIABLE_NON_VOLATILE.

EFI_VARIABLE_HARDWARE_ERROR_RECORD is always handled separately; it always
takes priority over the three cases listed above.

Introduce the GetMaxVariableSize() helper to consider
PcdMaxVolatileVariableSize, in addition to
GetNonVolatileMaxVariableSize(). GetNonVolatileMaxVariableSize() is
currently called at three sites, and two of those need to start using
GetMaxVariableSize() instead:
- VariableServiceInitialize() [VariableSmm.c]: the SMM comms buffer must
  accommodate all kinds of variables,
- VariableCommonInitialize() [Variable.c]: the preallocated scratch space
  must also accommodate all kinds of variables,
- InitNonVolatileVariableStore() [Variable.c] can continue using
  GetNonVolatileMaxVariableSize().

Don't modify the ReclaimForOS() function as it is specific to non-volatile
variables and should ignore PcdMaxVolatileVariableSize.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Gary Lin <glin@suse.com>
Tested-by: Gary Lin <glin@suse.com>
[lersek@redhat.com: set MaxVolatileVariableSize where Star suggested]
Reviewed-by: Star Zeng <star.zeng@intel.com>