Add ib_uverbs_flow_spec_gre to define a rule to match the GRE
encapsulation protocol.

The spec includes the generic specs header, type, size and reserved
fields while the filter itself is defined as ib_uverbs_flow_gre_filter
and includes:
1. Checksum present bit, key present bit and version bits in a single
   16bit field.
2. Protocol type field - Indicates the ether protocol type of the
   encapsulated payload.
3. Key field - present if key bit is set and contains an application
   specific key value.
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Ariel Levkovich <lariel@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>