f2fs: reuse make_empty_dir code for inline_dentry

This patch introduces do_make_empty_dir to mitigate code redundancy
for inline_dentry.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --cc fs/f2fs/dir.c
Simple merge

diff --cc fs/f2fs/f2fs.h
Simple merge

diff --cc fs/f2fs/inline.c
Simple merge