f2fs: use proper variable name

It is better to use variable name "inline_dentry" instead of "dentry_blk"
when data type is "struct f2fs_inline_dentry". This patch has no functional
changes, just to make code more readable especially when call the function
make_dentry_ptr_inline() and f2fs_convert_inline_dir().

Signed-off-by: Tiezhu Yang <kernelpatch@126.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>