fs: Use try_cmpxchg() in start_dir_add()

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 (and related
move instruction in front of CMPXCHG).

Note that the value from *ptr should be read using READ_ONCE() to
prevent the compiler from merging, refetching or reordering the read.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Link: https://lore.kernel.org/20250811125308.616717-1-ubizjak@gmail.com


Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>