ARM64: spinlock: Move the dsb+sev after the stlxr has succeeded

In arch_read_unlock, the earlier commit
88a06607 had added the DSB+SEV
before checking if the stlxr has successfully completed.
Move the DSB + SEV after the compare instruction to prevent
unnecessary SEVs.

Change-Id: I58bdad4db333a2acc31bcdc0e911c493cd3417bd
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>