erofs-utils: lib: fix get_unaligned_le64() return type

get_unaligned_le64() reads a 64-bit little-endian value but is
declared to return u32. Return u64 so callers do not lose the upper
32 bits.

Signed-off-by: Vansh Choudhary <ch@vnsh.in>
Link: https://lore.kernel.org/r/20260409173310.91143-1-ch@vnsh.in
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>