drm/panthor: harden firmware build-info bounds checks

panthor_fw_read_build_info() checks whether the metadata range fits in the
firmware image with hdr.meta_start + hdr.meta_size. Both fields are u32, so
the addition can wrap and let an out-of-bounds range pass validation.

The function also reads the "git_sha: " prefix without first checking that
the metadata is long enough, and meta_size == 0 can underflow the NULL
terminator index.

Use subtraction-based bounds checking and reject metadata that is too short
to contain the expected prefix and trailing NULL byte.

Fixes: 2718d91816ee ("drm/panthor: Add the FW logical block")
Cc: stable@vger.kernel.org
Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patch.msgid.link/20260720113212.11981-1-osama.abdelkader@gmail.com

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