compress: make Compression a regular non-sparse enum

Given we have two different types for the journal object flags and the
Compression enum, let's make the latter a regular non-sparse enum, and
thus remove some surprises. We have to convert anyway between the two,
and already do via COMPRESSION_FROM_OBJECT().

src/basic/compress.c

@@ -58,11 +58,10 @@ static int zstd_ret_to_errno(size_t ret) {
 #define ALIGN_8(l) ALIGN_TO(l, sizeof(size_t))
 
 static const char* const compression_table[_COMPRESSION_MAX] = {
+        [COMPRESSION_NONE] = "NONE",
         [COMPRESSION_XZ]   = "XZ",
         [COMPRESSION_LZ4]  = "LZ4",
         [COMPRESSION_ZSTD] = "ZSTD",
-        /* If we add too many more entries here, it's going to grow quite large (and be mostly sparse), since
-         * the array key is actually a bitmask, not a plain enum */
 };
 
 DEFINE_STRING_TABLE_LOOKUP(compression, Compression);

src/basic/compress.h

@@ -4,12 +4,10 @@
 #include <unistd.h>
 
 typedef enum Compression {
-        /* These are defined the same way as the relevant object types in journal-def.h,
-         * i.e. OBJECT_COMPRESSED_XZ, … */
-        COMPRESSION_NONE = 0,
-        COMPRESSION_XZ   = 1,
-        COMPRESSION_LZ4  = 2,
-        COMPRESSION_ZSTD = 4,
+        COMPRESSION_NONE,
+        COMPRESSION_XZ,
+        COMPRESSION_LZ4,
+        COMPRESSION_ZSTD,
         _COMPRESSION_MAX,
         _COMPRESSION_INVALID = -EINVAL,
 } Compression;