erofs-utils: lib: get rid of tmpfile()

erofs-utils: lib: get rid of tmpfile()

Currently, `tmpfile()` is problematic:

 - It uses `FILE *` instead of `fd`;
 - It may not leverage `$TMPDIR`, see `__gen_tempfd()`:
   https://sourceware.org/git?p=glibc.git;a=blob;f=stdio-common/tmpfile.c;hb=glibc-2.41

Switch to `erofs_tmpfile()` throughout the codebase.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250214062407.3281416-2-hsiangkao@linux.alibaba.com