locks: Fix dropped call to ->fl_release_private()

Prior to commit 4149be7b, sys_flock() would allocate the file_lock
struct it was going to use to pass parameters, call ->flock() and then call
locks_free_lock() to get rid of it - which had the side effect of calling
locks_release_private() and thus ->fl_release_private().

With commit 4149be7b, however, this is no longer the case: the struct
is now allocated on the stack, and locks_free_lock() is no longer called -
and thus any remaining private data doesn't get cleaned up either.

This causes afs flock to cause oops.  Kasan catches this as a UAF by the
list_del_init() in afs_fl_release_private() for the file_lock record
produced by afs_fl_copy_lock() as the original record didn't get delisted.
It can be reproduced using the generic/504 xfstest.

Fix this by reinstating the locks_release_private() call in sys_flock().
I'm not sure if this would affect any other filesystems.  If not, then the
release could be done in afs_flock() instead.

Changes
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