FAT incorrect macro

[resyfer]

In this, DIRSEC_BYTENDX takes in f and i, yet expansion only uses i.

Macro is used like DIRSEC_BYTENDX(fs, idx), which needs to expand to have DIRSEC_NDXMASK(fs), but due to the nature of the typo, it still works as intended as it uses fs right from the scope of the function instead when it expands.

[acassis]

Closed by #11647