arm64: compat: fix stxr failure case in SWP emulation

arm64: compat: fix stxr failure case in SWP emulation

If the STXR instruction fails in the SWP emulation code, we leave *data
overwritten with the loaded value, therefore corrupting the data written
by a subsequent, successful attempt.

This patch re-jigs the code so that we only write back to *data once we
know that the update has happened.

Cc: <[email protected]>
Fixes: bd35a4adc413 ("arm64: Port SWP/SWPB emulation support from arm")
Reported-by: Shengjiu Wang <[email protected]>
Reported-by: Vladimir Murzin <[email protected]>
Signed-off-by: Will Deacon <[email protected]>