Added usedBytes to match other filesystems (#4534)

Author: lbernstone

libraries/FFat/src/FFat.cpp

@@ -135,6 +135,21 @@ size_t F_Fat::totalBytes()
     return tot_sect * sect_size;
 }
 
+size_t F_Fat::usedBytes()
+{
+    FATFS *fs;
+    DWORD free_clust, used_sect, sect_size;
+
+    BYTE pdrv = ff_diskio_get_pdrv_wl(_wl_handle);
+    char drv[3] = {(char)(48+pdrv), ':', 0};
+    if ( f_getfree(drv, &free_clust, &fs) != FR_OK){
+        return 0;
+    }
+    used_sect = (fs->n_fatent - 2 - free_clust) * fs->csize;
+    sect_size = CONFIG_WL_SECTOR_SIZE;
+    return used_sect * sect_size;
+}
+
 size_t F_Fat::freeBytes()
 {
 

libraries/FFat/src/FFat.h

@@ -31,6 +31,7 @@ class F_Fat : public FS
     bool begin(bool formatOnFail=false, const char * basePath="/ffat", uint8_t maxOpenFiles=10, const char * partitionLabel = (char*)FFAT_PARTITION_LABEL);
     bool format(bool full_wipe = FFAT_WIPE_QUICK, char* partitionLabel = (char*)FFAT_PARTITION_LABEL);
     size_t totalBytes();
+    size_t usedBytes();
     size_t freeBytes();
     void end();
     bool exists(const char* path);