mshv: Use try_cmpxchg() instead of cmpxchg()

mshv: Use try_cmpxchg() instead of cmpxchg()

Use !try_cmpxchg() instead of cmpxchg (*ptr, old, new) != old.
x86 CMPXCHG instruction returns success in ZF flag, so this
change saves a compare after CMPXCHG.

The generated assembly code improves from e.g.:

     415:	48 8b 44 24 30       	mov    0x30(%rsp),%rax
     41a:	48 8b 54 24 38       	mov    0x38(%rsp),%rdx
     41f:	f0 49 0f b1 91 a8 02 	lock cmpxchg %rdx,0x2a8(%r9)
     426:	00 00
     428:	48 3b 44 24 30       	cmp    0x30(%rsp),%rax
     42d:	0f 84 09 ff ff ff    	je     33c <...>

to:

     415:	48 8b 44 24 30       	mov    0x30(%rsp),%rax
     41a:	48 8b 54 24 38       	mov    0x38(%rsp),%rdx
     41f:	f0 49 0f b1 91 a8 02 	lock cmpxchg %rdx,0x2a8(%r9)
     426:	00 00
     428:	0f 84 0e ff ff ff    	je     33c <...>

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Long Li <longli@microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>