use copy_file_range for file copy in mkcomposefs

[divineaugustine]

This change was done as part of multi-threading (#269 ) however, copy_file_range needs some fallback handling for file copy when copying files across different file systems (https://lwn.net/Articles/846403/). It is agreed that we rollback copy_file_range from multi-threading PR and raise a separate PR for just copy_file_range