xarray: extract xa_zero_to_null

Patch series "xarray: extract __xa_cmpxchg_raw".

This series reduces duplication between __xa_cmpxchg and __xa_insert by
extracting a new function that does not coerce zero entries to null on the
return path.

The new function may be used by the upcoming Rust xarray abstraction in
its reservation API where it is useful to tell the difference between zero
entries and null slots.


This patch (of 2):

Reduce code duplication by extracting a static inline function that
returns its argument if it is non-zero and NULL otherwise.

This changes xas_result to check for errors before checking for zero but
this cannot change the behavior of existing callers:
- __xa_erase: passes the result of xas_store(_, NULL) which cannot fail.
- __xa_store: passes the result of xas_store(_, entry) which may fail.
  xas_store calls xas_create when entry is not NULL which returns NULL
  on error, which is immediately checked. This should not change
  observable behavior.
- __xa_cmpxchg: passes the result of xas_load(_) which might be zero.
  This would previously return NULL regardless of the outcome of
  xas_store but xas_store cannot fail if xas_load returns zero
  because there is no need to allocate memory.
- xa_store_range: same as __xa_erase.

Link: https://lkml.kernel.org/r/20241112-xarray-insert-cmpxchg-v1-0-dc2bdd8c4136@gmail.com
Link: https://lkml.kernel.org/r/20241112-xarray-insert-cmpxchg-v1-1-dc2bdd8c4136@gmail.com


Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Andreas Hindborg <a.hindborg@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>