f2fs: introduce f2fs_dentry_ptr structure for code clean-up

This patch introduces f2fs_dentry_ptr structure for the use of a function
parameter in inline_dentry operations.

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