RDMA/core: Cosmetic change - move member initialization to correct block

old_pd is used only if IB_MR_REREG_PD flags is set.
For readability move it's initialization to where it is used.

While there rewrite the whole 'if-else' block so on error jump directly
to label and no need for 'else'

Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>