mmap-cache: add MMapFileDescriptor.cache accessor

Sometimes we want to reuse an existing MMapFileDescriptor's
cache, but it's a private struct.

This lets us access that pointer if necessary.

src/libsystemd/sd-journal/mmap-cache.c

@@ -591,3 +591,9 @@ void mmap_cache_fd_free(MMapFileDescriptor *f) {
 
         free(f);
 }
+
+MMapCache* mmap_cache_fd_cache(MMapFileDescriptor *f) {
+        assert(f);
+
+        return f->cache;
+}

src/libsystemd/sd-journal/mmap-cache.h

@@ -22,7 +22,8 @@ int mmap_cache_fd_get(
         size_t size,
         struct stat *st,
         void **ret);
-MMapFileDescriptor * mmap_cache_add_fd(MMapCache *m, int fd, int prot);
+MMapFileDescriptor* mmap_cache_add_fd(MMapCache *m, int fd, int prot);
+MMapCache* mmap_cache_fd_cache(MMapFileDescriptor *f);
 void mmap_cache_fd_free(MMapFileDescriptor *f);
 
 void mmap_cache_stats_log_debug(MMapCache *m);