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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>