lib-sieve: storage: file: Use container_of()

src/lib-sieve/storage/file/sieve-file-script-sequence.c

src/lib-sieve/storage/file/sieve-file-script.c

src/lib-sieve/storage/file/sieve-file-storage-active.c

src/lib-sieve/storage/file/sieve-file-storage-list.c

src/lib-sieve/storage/file/sieve-file-storage-quota.c

src/lib-sieve/storage/file/sieve-file-storage-save.c

src/lib-sieve/storage/file/sieve-file-storage.c