SYCL: reduce allocation overhead during flash attention (#22732)
* SYCL: reduce allocation overhead during flash attention
* tidy up whitespace
* add a note about the flag
* move ggml_sycl_fattn_* into fattn-buffers.hpp
* refactor implementation into fattn-buffers.cpp
* move new_fattn_kv_buffers back into ggml-sycl.cpp