cifs: fix skipping to incorrect offset in emit_cached_dirents

When application has done lseek() to a different offset on a directory fd
we skipped one entry too many before we start emitting directory entries
from the cache.

We need to also make sure that when we are starting to emit directory
entries from the cache, the ->pos sequence might have holes and skip
some indices.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reviewed-by: Tom Talpey <tom@talpey.com>
Signed-off-by: Steve French <stfrench@microsoft.com>