xarray: extract helper from __xa_{insert,cmpxchg}

Reduce code duplication by extracting a static inline function.  This
function is identical to __xa_cmpxchg with the exception that it does not
coerce zero entries to null on the return path.

[tamird@gmail.com: fix __xa_erase()]
  Link: https://lkml.kernel.org/r/CAJ-ks9kN_qddZ3Ne5d=cADu5POC1rHd4rQcbVSD_spnZOrLLZg@mail.gmail.com
Link: https://lkml.kernel.org/r/20241112-xarray-insert-cmpxchg-v1-2-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>