#100 Apply simplefs_writepage() only for kernels under 6.16

aops->writepage has been removed and replaced with writepages since 6.16. The current codebase still
builds with simplefs_writepage() on kernels higher than 6.15, which gives unused-function warning.

Add in macros to ensure kernel module builds with simplefs_writepage() for kernels under 6.16.

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Summary by cubic

Fixes a build warning on kernels 6.16 and newer by compiling simplefs_writepage() only for kernels under 6.16.

Kernel 6.16 removed aops->writepage in favor of writepages, so the function was previously built but unused, triggering an unused-function warning.