VFS: Convert file->f_dentry->d_inode to file_inode()

Convert file->f_dentry->d_inode to file_inode() so as to get layered
filesystems right.

Found with: git grep '[.>]f_dentry'

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>