locks: make posix_test_lock() interface more consistent

Since posix_test_lock(), like fcntl() and ->lock(), indicates absence or
presence of a conflict lock by setting fl_type to, respectively, F_UNLCK
or something other than F_UNLCK, the return value is no longer needed.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>