Replace a bunch of file->dentry->d_inode refs with file_inode().

In __fput(), use file->f_inode instead so as not to be affected by any tricks
that file_inode() might grow.
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>