[XFS] Initialise current offset in xfs_file_readdir correctly

[XFS] Initialise current offset in xfs_file_readdir correctly

After reading the directory contents into the temporary buffer, we grab
each dirent and pass it to filldir witht eh current offset of the dirent.
The current offset was not being set for the first dirent in the temporary
buffer, which coul dresult in bad offsets being set in the f_pos field
result in looping and duplicate entries being returned from readdir.

SGI-PV: 974905
SGI-Modid: xfs-linux-melb:xfs-kern:30282a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>