NTFS: Allow highmem kmalloc() in ntfs_malloc_nofs() and add _nofail() version.

- Modify fs/ntfs/malloc.h::ntfs_malloc_nofs() to do the kmalloc() based
  allocations with __GFP_HIGHMEM, analogous to how the vmalloc() based
  allocations are done.
- Add fs/ntfs/malloc.h::ntfs_malloc_nofs_nofail() which is analogous to
  ntfs_malloc_nofs() but it performs allocations with __GFP_NOFAIL and
  hence cannot fail.

Signed-off-by: Anton Altaparmakov <aia21@cantab.net>