vfs_fat: allocate FIL structures on the heap in vfs_fat_link

vfs_fat: allocate FIL structures on the heap in vfs_fat_link

vfs_fat_link opened two files to perform copy operation. File structures
were allocated on the stack. When _MAX_SS setting was increased in
ffconf.h due to wear levelling feature, the size of these structures
increased to ~4k each (~8k total). This exceeds stack size allocated for
tasks in most typical cases.

This change makes file structures dynamically allocated.