pyverbs: Extend MR's write method

The current implementation of write() doesn't take an offset parameter.
In UD this means that extra 40 bytes need to be written, so that the
GRH won't override the beginning of the message.
Add offset parameter to write().

Signed-off-by: Maxim Chicherin <maximc@mellanox.com>
Signed-off-by: Edward Srouji <edwards@mellanox.com>

pyverbs/mr.pyx

@@ -129,21 +129,23 @@ cdef class MR(PyverbsCM):
             self.pd = None
             self.buf = NULL
 
-    def write(self, data, length):
+    def write(self, data, length, offset=0):
         """
         Write user data to the MR's buffer using memcpy
         :param data: User data to write
         :param length: Length of the data to write
+        :param offset: Writing offset
         :return: None
         """
         if not self.buf or length < 0:
             raise PyverbsUserError('The MR buffer isn\'t allocated or length'
                                    f' {length} is invalid')
         # If data is a string, cast it to bytes as Python3 doesn't
         # automatically convert it.
+        cdef int off = offset
         if isinstance(data, str):
             data = data.encode()
-        memcpy(self.buf, <char *>data, length)
+        memcpy(<char*>(self.buf + off), <char *>data, length)
 
     cpdef read(self, length, offset):
         """