tests: fix problems in the m_rootgnutar test #74

Job	Run time
Build and test with ASAN enabled	1m 34s
Build and test with gcc	51s
Build and test with clang	1m 13s
Cross-compile for Android (aarch64)	37s
Build and test with gcc -m32	1m 10s
Cross-compile for Android (armv7a)	37s
Build and test on macOS	1m 48s
Cross-compile for Android (i686)	45s
Build and test with UBSAN enabled	1m 19s
Build mke2fs on Windows with mingw32	2m 19s
Cross-compile for Android (x86_64)	52s
Build mke2fs on Windows with mingw64	2m 19s
	15m 24s

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