ntfs: fix mmap_prepare writable check for shared mappings

ntfs: fix mmap_prepare writable check for shared mappings

Linus pointed out that checking only VMA_WRITE_BIT is incorrect.
Private writable mappings (MAP_PRIVATE) set VM_WRITE but do not
write back to the filesystem. Also, mappings that can become
writable via mprotect() (VM_MAYWRITE) must be handled.

Use vma_desc_test_all(VMA_SHARED_BIT, VMA_MAYWRITE_BIT) instead,
which matches what other filesystems do.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>