netfs: Mark __nonstring lookup tables

netfs: Mark __nonstring lookup tables

GCC 15's new -Wunterminated-string-initialization notices that the
character lookup tables "fscache_cache_states" and "fscache_cookie_states"
(which are not used as a C-String) need to be marked as "nonstring":

fs/netfs/fscache_cache.c:375:67: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (6 chars into 5 available) [-Wunterminated-string-initialization]
  375 | static const char fscache_cache_states[NR__FSCACHE_CACHE_STATE] = "-PAEW";
      |                                                                   ^~~~~~~
fs/netfs/fscache_cookie.c:32:69: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (11 chars into 10 available) [-Wunterminated-string-initialization]
   32 | static const char fscache_cookie_states[FSCACHE_COOKIE_STATE__NR] = "-LCAIFUWRD";
      |                                                                     ^~~~~~~~~~~~

Annotate the arrays.

Signed-off-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/20250416221654.work.028-kees@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>