Fix version-check macro precedence and collapse write_begin variants

[RoyWFHuang]

1. Add missing parentheses to version-check macros

SIMPLEFS_AT_LEAST() and SIMPLEFS_LESS_EQUAL() lacked outer parentheses:

#define SIMPLEFS_AT_LEAST(major, minor, rev)  LINUX_VERSION_CODE >= KERNEL_VERSION(major, minor, rev)

Unary ! binds tighter than >=, so #if !SIMPLEFS_AT_LEAST(6, 15, 0) in file.c expanded to #if (!LINUX_VERSION_CODE) >= KERNEL_VERSION(6, 15, 0), which is false on every kernel. .writepage was dropped from simplefs_aops everywhere, not just on 6.15+.

2. Collapse duplicated simplefs_write_begin variants

The four per-kernel variants each carried a full copy of the same body, differing only in signature and in the arguments to block_write_begin().

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Summary by cubic

Fixes version-check macro precedence and deduplicates simplefs_write_begin. Restores correct kernel gating (e.g., .writepage only dropped on 6.15+) and reduces maintenance.

• Bug Fixes

  • Parenthesized SIMPLEFS_AT_LEAST() and SIMPLEFS_LESS_EQUAL() to enforce correct precedence.
  • Fixed #if !SIMPLEFS_AT_LEAST(6, 15, 0) mis-evaluation that dropped .writepage on all kernels.

• Refactors

  • Collapsed four kernel-specific simplefs_write_begin variants into one implementation.
  • Kept a single body; only the block_write_begin() call/signature is selected via version guards.

^{Written for commit 65bec5b. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[jserv]

Thank @RoyWFHuang for contributing!