um: hostfs: Reduce number of syscalls in readdir

Currently hostfs issues every time a seekdir(), in fact
it has to do this only upon the first call.
Also telldir() can be omitted as we can obtain the directory
offset from readdir().

Signed-off-by: Richard Weinberger <richard@nod.at>

diff --cc fs/hostfs/hostfs.h
Simple merge

diff --cc fs/hostfs/hostfs_kern.c
Simple merge

diff --cc fs/hostfs/hostfs_user.c
Simple merge