f2fs: fix to use highmem for pages of newly created directory

f2fs: fix to use highmem for pages of newly created directory

In commit e580e5370da6 ("f2fs: use highmem for directory pages"), we have set
__GFP_HIGHMEM into dir mapping's gfp flag in f2fs_iget, so high address memory
could be used for these existing dir's page.

But we forgot to set flag for newly created dir, due to this reason, our newly
created dir pages could not be allocated from high address memory. Fix it.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>