libext2fs: fix a coding style for EXT2_NO_MTAB_FILE

When we define an error in lib/ext2fs/ext2_err.et.in, we will always use
EXT2_ET_* prefix for a new error.  But EXT2_NO_MTAB_FILE doesn't obey
this rule.  So fix it.

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/lib/ext2fs/ext2_err.et.in b/lib/ext2fs/ext2_err.et.in
index d20c6b7..de1b05b 100644
--- a/lib/ext2fs/ext2_err.et.in
+++ b/lib/ext2fs/ext2_err.et.in
@@ -416,7 +416,7 @@
 ec	EXT2_ET_IO_CHANNEL_NO_SUPPORT_64,
 	"I/O Channel does not support 64-bit block numbers"
 
-ec	EXT2_NO_MTAB_FILE,
+ec	EXT2_ET_NO_MTAB_FILE,
 	"Can't check if filesystem is mounted due to missing mtab file"
 
 ec	EXT2_ET_CANT_USE_LEGACY_BITMAPS,
diff --git a/lib/ext2fs/ismounted.c b/lib/ext2fs/ismounted.c
index 9bccc74..2c1bd75 100644
--- a/lib/ext2fs/ismounted.c
+++ b/lib/ext2fs/ismounted.c
@@ -58,7 +58,7 @@
 			if (getenv("EXT2FS_NO_MTAB_OK"))
 				return 0;
 			else
-				return EXT2_NO_MTAB_FILE;
+				return EXT2_ET_NO_MTAB_FILE;
 		}
 		return errno;
 	}