Revert "df: remove the unused fstype var"

This reverts commit 647f70e6bf4213f3a892277fcafe2c36d81aa3a5.

fstype is used in a call to build_iovec_argf() in this branch, and the
reverted commit didn't remove the variable, just its initialization.

This is a direct commit to stable/12.

Reported by: GCC -Wmaybe-uninitialized