xfs: fix the wrong new_size/rnew_size at xfs_iext_realloc_direct()

xfs: fix the wrong new_size/rnew_size at xfs_iext_realloc_direct()

At xfs_iext_realloc_direct(), the new_size is changed by adding
if_bytes if originally the extent records are stored at the inline
extent buffer, and we have to switch from it to a direct extent
list for those new allocated extents, this is wrong.

This patch fix above problem and revise the new_size comments at
xfs_iext_realloc_direct() to make it more readable.  Also, fix the
comments while switching from the inline extent buffer to a direct
extent list to reflect this change.

Ported from kernel commit 17ec81c1.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>