Adapted from wrapfs
commit 1d1d23a47baa ("Wrapfs: fix ->llseek to update upper and lower
offsets")

Fixes bug: xfstests generic/257. f_pos consistently is required by and
only by dir_ops->wrapfs_readdir, main_ops is not affected.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>
Signed-off-by: Mengyang Li <li.mengyang@stonybrook.edu>
Signed-off-by: Daniel Rosenberg <drosen@google.com>
Bug: 35766959
Change-Id: I360a1368ac37ea8966910a58972b81504031d437