Coverity analysis defect 134909: Macro compares unsigned to 0

Macro compares unsigned to 0

An unsigned value can never be negative, so this test will always evaluate the same way. An unsigned value can never be less than 0
/src/mongo/db/s/migration_chunk_cloner_source.cpp:1381: NO_EFFECT 134909 This greater-than-or-equal-to-zero comparison of an unsigned value is always true. "this->_inProgressReads >= 0UL".