Fix sign-extention problem with ll_extent_lock() error return codes on 64-bit

Fix sign-extention problem with ll_extent_lock() error return codes on 64-bit
arches.  They were being stored in an ldlm_error_t (an enum, apparently an
unsigned int) instead of an int (ll_extent_lock() does not return LDLM error
codes), and when returned in a 64-bit ssize_t they were 2^32 - errno instead
of a negative number.
b=3043