lib/test_lockup.c: fix return value of test_lockup_init()

lib/test_lockup.c: fix return value of test_lockup_init()

Since filp_open() returns an error pointer, we should use IS_ERR() to
check the return value and then return PTR_ERR() if failed to get the
actual return value instead of always -EINVAL.

E.g. without this patch:

[root@localhost loongson]# ls no_such_file
ls: cannot access no_such_file: No such file or directory
[root@localhost loongson]# modprobe test_lockup file_path=no_such_file lock_sb_umount time_secs=60 state=S
modprobe: ERROR: could not insert 'test_lockup': Invalid argument
[root@localhost loongson]# dmesg | tail -1
[  126.100596] test_lockup: cannot find file_path

With this patch:

[root@localhost loongson]# ls no_such_file
ls: cannot access no_such_file: No such file or directory
[root@localhost loongson]# modprobe test_lockup file_path=no_such_file lock_sb_umount time_secs=60 state=S
modprobe: ERROR: could not insert 'test_lockup': Unknown symbol in module, or unknown parameter (see dmesg)
[root@localhost loongson]# dmesg | tail -1
[   95.134362] test_lockup: failed to open no_such_file: -2

Fixes: 5908688da884 ("lib/test_lockup.c: add parameters for locking generic vfs locks")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Kees Cook <keescook@chromium.org>
Link: http://lkml.kernel.org/r/1595555407-29875-2-git-send-email-yangtiezhu@loongson.cn
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>