Took advantage of file buffer layout to pass as lfsr_data_t directly

This would have been more valuable if the extra lfsr_data_t stack
allocation (12 bytes) wasn't already unioned with the btree's encoding
buffer allocation (18 bytes):

           code          stack
  before: 33714           2640
  after:  33702 (-0.0%)   2640 (+0.0%)

Oh well, this still might save some stack in the future if things shift
around.

lfs.h

@@ -519,10 +519,11 @@ typedef struct lfsr_file {
     lfsr_bshrub_t bshrub_;
     lfs_off_t pos;
 
+    // note this lines up with lfsr_data_t's buffer representation
     struct {
-        lfs_off_t pos;
         lfs_off_t size;
         uint8_t *buffer;
+        lfs_off_t pos;
     } buffer;
 
     lfs_block_t eblock;