f2fs: avoid punch_hole overhead when releasing volatile data

This patch is to avoid some punch_hole overhead when releasing volatile data.
If volatile data was not written yet, we just can make the first page as zero.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index dbe1144..9e37817 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -1232,6 +1232,7 @@
 	FI_NEED_IPU,		/* used for ipu per file */
 	FI_ATOMIC_FILE,		/* indicate atomic file */
 	FI_VOLATILE_FILE,	/* indicate volatile file */
+	FI_FIRST_BLOCK_WRITTEN,	/* indicate #0 data block was written */
 	FI_DROP_CACHE,		/* drop dirty page cache */
 	FI_DATA_EXIST,		/* indicate data exists */
 };
@@ -1340,6 +1341,11 @@
 	return is_inode_flag_set(F2FS_I(inode), FI_VOLATILE_FILE);
 }
 
+static inline bool f2fs_is_first_block_written(struct inode *inode)
+{
+	return is_inode_flag_set(F2FS_I(inode), FI_FIRST_BLOCK_WRITTEN);
+}
+
 static inline bool f2fs_is_drop_cache(struct inode *inode)
 {
 	return is_inode_flag_set(F2FS_I(inode), FI_DROP_CACHE);