x86/spec_ctrl: Only set SPEC_CTRL_IBRS_FIRMWARE if IBRS is actually in use

x86/spec_ctrl: Only set SPEC_CTRL_IBRS_FIRMWARE if IBRS is actually in use

Currently the SPEC_CTRL_IBRS_FIRMWARE flag always gets set as long as
IBRS is supported by the hardware. However, as best as can be determined
by the documention, if IBRS has been disabled (e.g., spectre_v2=off) then
SPEC_CTRL_IBRS_FIRMWARE should not be set:

nospectre_v2	[X86] Disable all mitigations for the Spectre variant 2
		(indirect branch prediction) vulnerability. System may
		allow data leaks with this option, which is equivalent
		to spectre_v2=off.

and:

spectre_v2=	[X86] Control mitigation of Spectre variant 2
		(indirect branch speculation) vulnerability.

		off  - unconditionally disable

Add a check in set_ibrs_firmware() to only set SPEC_CTRL_IBRS_FIRMWARE if
ibrs_disabled is not also set.

Orabug: 28274907

Signed-off-by: Patrick Colp <patrick.colp@oracle.com>
Reviewed-by: Kanth Ghatraju <kanth.ghatraju@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>