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>
diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index c43262d..67da4f6 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -1044,7 +1044,7 @@
 			struct buffer_head *bh_result, int create)
 {
 	return __get_data_block(inode, iblock, bh_result, create,
-						F2FS_GET_BLOCK_DIO, NULL);
+						F2FS_GET_BLOCK_DEFAULT, NULL);
 }
 
 static int get_data_block_bmap(struct inode *inode, sector_t iblock,
@@ -1244,7 +1244,7 @@
 			map.m_len = last_block - block_in_file;
 
 			if (f2fs_map_blocks(inode, &map, 0,
-						F2FS_GET_BLOCK_READ))
+						F2FS_GET_BLOCK_DEFAULT))
 				goto set_error_page;
 		}
 got_it: