arm64/fpsimd: Reset FPMR upon exec()

An exec() is expected to reset all FPSIMD/SVE/SME state, and barring
special handling of the vector lengths, the state is expected to reset
to zero. This reset is handled in fpsimd_flush_thread(), which the core
exec() code calls via flush_thread().

When support was added for FPMR, no logic was added to
fpsimd_flush_thread() to reset the FPMR value, and thus it is
erroneously inherited across an exec().

Add the missing reset of FPMR.

Fixes: 203f2b95a8 ("arm64/fpsimd: Support FEAT_FPMR")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250409164010.3480271-9-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/fpsimd.c

@@ -1638,6 +1638,9 @@ void fpsimd_flush_thread(void)
 		current->thread.svcr = 0;
 	}
 
+	if (system_supports_fpmr())
+		current->thread.uw.fpmr = 0;
+
 	current->thread.fp_type = FP_STATE_FPSIMD;
 
 	put_cpu_fpsimd_context();