hostfs: Use correct mask for file mode

S_IFMT is obviously wrong and needs to be 0777.
We're interested in the file mode, not the type.

Fixes: b98b9102

 (hostfs: No need to box and later unbox the file mode)
Reported-by: Markus Stenberg <markus.stenberg@iki.fi>
Signed-off-by: Richard Weinberger <richard@nod.at>