mmu: save flags passed during file mapping in a vma.

Currently flags are saved for anon_vma, but not for file_vma. Examining
those flags in gdb proved to be helpful. Remove _shared since it no
longer needed, the information is in the flags.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>