NTFS: - Fix two race conditions in fs/ntfs/inode.c::ntfs_put_inode().

NTFS: - Fix two race conditions in fs/ntfs/inode.c::ntfs_put_inode().

- Fix race condition in fs/ntfs/inode.c::ntfs_put_inode() by moving the
  index inode bitmap inode release code from there to
  fs/ntfs/inode.c::ntfs_clear_big_inode().  (Thanks to Christoph
  Hellwig for spotting this.)
- Fix race condition in fs/ntfs/inode.c::ntfs_put_inode() by taking the
  inode semaphore around the code thst sets ni->itype.index.bmp_ino to
  NULL and reorganize the code to optimize it a bit.  (Thanks to
  Christoph Hellwig for spotting this.)

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