posix_test_lock() and ->lock() do the same job but have gratuitously
different interfaces.  Modify posix_test_lock() so the two agree,
simplifying some code in the process.
Signed-off-by: NMarc Eshel <eshel@almaden.ibm.com>
Signed-off-by: N"J. Bruce Fields" <bfields@citi.umich.edu>