MIPS: memset.S: Fix return of __clear_user from Lpartial_fixup

MIPS: memset.S: Fix return of __clear_user from Lpartial_fixup

The __clear_user function is defined to return the number of bytes that
could not be cleared. From the underlying memset / bzero implementation
this means setting register a2 to that number on return. Currently if a
page fault is triggered within the memset_partial block, the value
loaded into a2 on return is meaningless.

The label .Lpartial_fixup\@ is jumped to on page fault. In order to work
out how many bytes failed to copy, the exception handler should find how
many bytes left in the partial block (andi a2, STORMASK), add that to
the partial block end address (a2), and subtract the faulting address to
get the remainder. Currently it incorrectly subtracts the partial block
start address (t1), which has additionally been clobbered to generate a
jump target in memset_partial. Fix this by adding the block end address
instead.

This issue was found with the following test code:
      int j, k;
      for (j = 0; j < 512; j++) {
        if ((k = clear_user(NULL, j)) != j) {
           pr_err("clear_user (NULL %d) returned %d\n", j, k);
        }
      }
Which now passes on Creator Ci40 (MIPS32) and Cavium Octeon II (MIPS64).

Suggested-by: James Hogan <jhogan@kernel.org>
Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: stable@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/19108/
Signed-off-by: James Hogan <jhogan@kernel.org>