fs: fix lock initialization

locks_alloc_lock() assumed that the allocated struct file_lock is
already initialized to zero members.  This is only true for the first
allocation of the structure, after reuse some of the members will have
random values.

This will for example result in passing random fl_start values to
userspace in fuse for FL_FLOCK locks, which is an information leak at
best.

Fix by reinitializing those members which may be non-zero after freeing.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
CC: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>