Fix GC bug in filelock.c

Fix GC bug in filelock.c

Fix a bug where if GC occurred at the wrong moment when locking a
file, the lock file’s name was trashed so file locking did not work.
This bug was introduced in Emacs 28.1.  The bug sometimes caused
filelock-tests-detect-external-change test failures on Fedora 35
x86-64 in an en_US.utf8 locale.
* src/filelock.c (lock_file_1, current_lock_owner, lock_if_free)
(lock_file, unlock_file, Ffile_locked_p):
Use Lisp_Object, not char *, for string, so that GC doesn’t trash
string contents.
(make_lock_file_name): Return the encoded name, not the original.
All callers changed.