f2fs: merge equivalent flags F2FS_GET_BLOCK_[READ|DIO]

Currently, the two flags F2FS_GET_BLOCK_[READ|DIO] are totally equivalent
and can be used interchangably in all scenarios they are involved in.
Neither of the flags is referenced in f2fs_map_blocks(), making them both
the default case. To remove the ambiguity, this patch merges both flags
into F2FS_GET_BLOCK_DEFAULT, and introduces an enum for all distinct flags.

Signed-off-by: Qiuyang Sun <sunqiuyang@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>