internal/luks2: acquireSharedLock should make more consistent use of `stat`/`fstat`

This code is currently performing a mix of `stat` and `fstat` calls via go's `os` package (which give us a `os.FileInfo` implementation) and `stat` and `fstat` system calls via the `unix` package which give is a `unix.Stat_t`. We should investigate using only one set of these - AFAICT, the Linux implementations of `os.FileInfo` return a `syscall.Stat_t` when calling `os.FileInfo.Sys` which would remove the need for the direct use of system calls, but this needs further investigation.