switch hostfs_iattr to explicit unsigned short

It's shared between kernel-compiled hostfs_kern and userland-compiled
hostfs_user (it's uml stuff).  Use explicit type instead of playing
silly buggers with mode_t.  It's not a userland API per se; it interacts
between code compiled with types same as for host kernel and, directly
linked to it, code talking to libc.  Both sides come from the same
kernel source...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>