file_remove_suid() is a generic function operates on struct file,
it almost has no relations with file mapping, so move it to fs/inode.c.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NCong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>