* Right-shift the values in GET_FBD_FAT_IDX and GET_FBD_NF_IDX, so
that the callers get the result they expect.
* Fix definition of FERR_FAT_FBD_ERR_MASK.
* Call GET_FBD_NF_IDX, not GET_FBD_FAT_IDX, when operating on
register FERR_NF_FBD. We were lucky they have the same definition.
This fixes kernel bug #44131:
https://bugzilla.kernel.org/show_bug.cgi?id=44131