squash! arm64/sve: Signal handling support

Signed-off-by: Dave Martin <Dave.Martin@arm.com>

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Changes since v4
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Requested by Will Deacon:

 * Fix inconsistent return semantics in restore_sve_fpsimd_context().

   Currently a nonzero return value from __copy_from_user() is passed
   back as-is to the caller or restore_sve_fpsimd_context(), rather than
   translating it do an error code as is done elsewhere.

   Callers of restore_sve_fpsimd_context() only care whether the return
   value is 0 or not, so this isn't a big deal, but it is more
   consistent to return proper error codes on failure in case we grow a
   use for them later.

   This patch returns -EFAULT in the __copy_from_user() error cases
   that weren't explicitly handled.