cmake: revert librados_tp.so version from 3 to 2

cmake: revert librados_tp.so version from 3 to 2

Post-mortem analysis:

librados.so and librados_tp.so are packaged together in the librados2
RPM.

c680fb10f598267f15a37fdb7bc44529c37c9318 bumped
the librados.so and librados_tp.so versions from 2 to 3.

Later, 7bf6b5ee1208a359826c74ab033e6bbbfc65969f reverted the librados.so
version from 3 to 2, but left the librados_tp.so version at 3.

Fixes: http://tracker.ceph.com/issues/39291
Signed-off-by: Nathan Cutler <ncutler@suse.com>
(cherry picked from commit b555ba6e2b99400937264edbd3031fcf24fd9005)