Fixed issues with reading and caching inline files

Kind of a two-fold issue. One, the programming to the middle of inline
files was causing the cache to get updated to a half programmed state.
While fine, as all programs do occur in order in a block, this is less
efficient when writing to inline files as it would cause the inline file
to need to be reread even if it fits in the cache.

Two, the rereading of the inline file was broken and passed the file's
tag all the way to where a user would expect an error. This was easy to
fix but adds to the reasons we should have test coverage information.

Found by ebinans

lfs.c

@@ -194,7 +194,7 @@ static int lfs_bd_prog(lfs_t *lfs,
             off += diff;
             size -= diff;
 
-            pcache->size = off - pcache->off;
+            pcache->size = lfs_max(pcache->size, off - pcache->off);
             if (pcache->size == lfs->cfg->cache_size) {
                 // eagerly flush out pcache if we fill up
                 int err = lfs_bd_flush(lfs, pcache, rcache, validate);
@@ -617,7 +617,7 @@ static int lfs_dir_getread(lfs_t *lfs, const lfs_mdir_t *dir,
                 lfs->cfg->cache_size);
         int err = lfs_dir_getslice(lfs, dir, gmask, gtag,
                 rcache->off, rcache->buffer, rcache->size);
-        if (err) {
+        if (err < 0) {
             return err;
         }
     }

tests/test_seek.sh

@@ -395,6 +395,16 @@ tests/test.py << TEST
         lfs_file_sync(&lfs, &file[0]) => 0;
         lfs_file_tell(&lfs, &file[0]) => i+1;
         lfs_file_size(&lfs, &file[0]) => $SIZE;
+        if (i < $SIZE-2) {
+            uint8_t c[3];
+            lfs_file_seek(&lfs, &file[0], -1, LFS_SEEK_CUR) => i;
+            lfs_file_read(&lfs, &file[0], &c, 3) => 3;
+            lfs_file_tell(&lfs, &file[0]) => i+3;
+            lfs_file_size(&lfs, &file[0]) => $SIZE;
+            lfs_file_seek(&lfs, &file[0], i+1, LFS_SEEK_SET) => i+1;
+            lfs_file_tell(&lfs, &file[0]) => i+1;
+            lfs_file_size(&lfs, &file[0]) => $SIZE;
+        }
     }
 
     lfs_file_seek(&lfs, &file[0], 0, LFS_SEEK_SET) => 0;