Rename get_operation_entry() to get_op_entry()
filesystem: remove unneeded template of ctor
ops_type_db: throw an exception in get_ops()
default_file_op: read: Fix bug causing SEGV

 In default_file_op::read(), if `offset` value is
larger than file size, then out of bounds of read is occurred.
 Fix this by returning 0 if `offset` is larger than file size.
Implement chmod functionality
st_inode: return const reference to a time
Add chown support