Using wrong filling value may cause unneeded extra bit rewriting.

Fix: use proper value to fill uneven bits.


mysql-test/r/type_bit.result:
  Fix for bug #22271: data casting may affect data stored in the next column(s?)
    - test case.
mysql-test/t/type_bit.test:
  Fix for bug #22271: data casting may affect data stored in the next column(s?)
    - test result.
sql/field.cc:
  Fix for bug #22271: data casting may affect data stored in the next column(s?)
    - use ((1 << bit_len) - 1) instead of 0xff to fill uneven bits 
      in order not to change other's bits.