arm64/sve: ptrace: Fix SVE_PT_REGS_OFFSET definition

SVE_PT_REGS_OFFSET is supposed to indicate the offset for skipping
over the ptrace NT_ARM_SVE header (struct user_sve_header) to the
start of the SVE register data proper.

However, currently SVE_PT_REGS_OFFSET is defined in terms of struct
sve_context, which is wrong: that structure describes the SVE
header in the signal frame, not in the ptrace regset.

This patch fixes the definition to use the ptrace header structure
struct user_sve_header instead.

By good fortune, the two structures are the same size anyway, so
there is no functional or ABI change.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>