riscv64: skip satp Bare-mode switch when already Bare (§V.3 item 7 audit fix)

arch_early_init() unconditionally cleared satp on every boot, justified
only by behavior observed under QEMU/EDK2 firmware (satp.MODE=10/Sv57,
kernel identity-mapped within it). That reasoning never applied to the
native U-Boot+OpenSBI boot path, where satp is conventionally already 0
at S-mode handoff -- the unconditional clear was likely a harmless no-op
there, but on an unverified assumption.

Fix: read satp.MODE first and return early when it's already 0 (nothing
to switch away from, no safety argument needed). The unconditional
csrw/sfence pair still runs unchanged for the confirmed QEMU/EDK2 case.
No Sv39/Sv48/Sv57 page-table walker built -- out of proportion to this
finding's severity.

3-arch acceptance: amd64/aarch64 don't compile this file, so their runs
are non-regression on untouched files only. riscv64's own boot log
confirms satp.MODE = 0xa at entry, so the mode != 0 branch ran and
"satp cleared -- Bare mode, explicit" printed exactly as before the fix.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019YcT3H2PQeyujrzjqS3Var

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