journal: Correctly advance offset when iterating hash table entries

pread() is not guaranteed to completely fill up the given buffer with
data which we assumed until now. Instead, only increment the offset by
the number of bytes that were actually read.

src/journal/journald-file.c

@@ -81,6 +81,7 @@ static int journald_file_punch_holes(JournalFile *f) {
         HashItem items[4096 / sizeof(HashItem)];
         uint64_t p, sz;
         size_t to_read;
+        ssize_t n;
         int r;
 
         r = journald_file_entry_array_punch_hole(
@@ -92,14 +93,12 @@ static int journald_file_punch_holes(JournalFile *f) {
         sz = le64toh(f->header->data_hash_table_size);
         to_read = MIN((size_t) f->last_stat.st_blksize, sizeof(items));
 
-        for (uint64_t i = p; i < p + sz; i += sizeof(items)) {
-                ssize_t n_read;
+        for (uint64_t i = p; i < p + sz; i += n) {
+                n = pread(f->fd, items, MIN(to_read, p + sz - i), i);
+                if (n < 0)
+                        return n;
 
-                n_read = pread(f->fd, items, MIN(to_read, p + sz - i), i);
-                if (n_read < 0)
-                        return n_read;
-
-                for (size_t j = 0; j < (size_t) n_read / sizeof(HashItem); j++) {
+                for (size_t j = 0; j < (size_t) n / sizeof(HashItem); j++) {
                         Object o;
 
                         for (uint64_t q = le64toh(items[j].head_hash_offset); q != 0;