target/arm: Ignore SMCR_EL2.LEN and SVCR_EL2.LEN if EL2 is not enabled

target/arm: Ignore SMCR_EL2.LEN and SVCR_EL2.LEN if EL2 is not enabled

When determining the current vector length, the SMCR_EL2.LEN and
SVCR_EL2.LEN settings should only be considered if EL2 is enabled
(compare the pseudocode CurrentSVL and CurrentNSVL which call
EL2Enabled()).

We were checking against ARM_FEATURE_EL2 rather than calling
arm_is_el2_enabled(), which meant that we would look at
SMCR_EL2/SVCR_EL2 when in Secure EL1 or Secure EL0 even if Secure EL2
was not enabled.

Use the correct check in sve_vqm1_for_el_sm().

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240722172957.1041231-5-peter.maydell@linaro.org
(cherry picked from commit f573ac059ed060234fcef4299fae9e500d357c33)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>