fix(kmod): resolve remaining checkpatch warnings

[Koichi98]

Resolve remaining checkpatch.pl warnings in agnocast_kmod/. The checkpatch CI was introduced in #1251 and EMBEDDED_FUNCTION_NAME was fixed in #1252. The following logic-related warnings still remain:

Remaining warnings

• SPDX_LICENSE_TAG (9): Missing SPDX license identifier
• OOM_MESSAGE (7): Unnecessary OOM messages (kernel already logs them)
• NEEDLESS_IF (3): if (p) kfree(p) — kfree(NULL) is safe
• MEMORY_BARRIER (3): Memory barriers without explanatory comments
• CONSTANT_COMPARISON (3): Yoda-style comparisons (0 == x → x == 0)
• ALLOC_WITH_MULTIPLY (2): Use kvcalloc instead of kvzalloc with multiply
• GLOBAL_INITIALISERS (2): Remove redundant = 0 / = false for globals
• CONST_STRUCT (1): struct file_operations should be const

Once all warnings are resolved, the checkpatch workflow should be made required

Reference: https://docs.kernel.org/dev-tools/checkpatch.html