locking/x86/xen: Use try_cmpxchg() in xen_alloc_p2m_entry()

Use try_cmpxchg() instead of cmpxchg(*ptr, old, new) == old.

The x86 CMPXCHG instruction returns success in the ZF flag,
so this change saves a compare after CMPXCHG.

Also, try_cmpxchg() implicitly assigns old *ptr value to "old"
when CMPXCHG fails. There is no need to explicitly assign
old *ptr value to the temporary, which can simplify the
surrounding source code.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20240405083335.507471-1-ubizjak@gmail.com


Signed-off-by: Juergen Gross <jgross@suse.com>