[CIFS] add local struct inode pointer to cifs_setattr

Clean up cifs_setattr a bit by adding a local inode pointer, and
changing all of the direntry->d_inode references to it. This also adds a
bit of micro-optimization. d_inode shouldn't change over the life of
this function, so we only need to dereference it once.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>