e2fsck: fix various -Wall nits picked up by clang #53

Job	Run time
Build and test with ASAN enabled	2m 6s
Build and test with gcc	52s
Cross-compile for Android (aarch64)	40s
Build and test with clang	1m 22s
Cross-compile for Android (armv7a)	35s
Build and test with gcc -m32	1m 19s
Cross-compile for Android (i686)	40s
Build and test on macOS	1m 55s
Cross-compile for Android (x86_64)	51s
Build and test with UBSAN enabled	1m 27s
Build mke2fs on Windows with mingw32	2m 18s
Build mke2fs on Windows with mingw64	2m 13s
	16m 18s

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