locks: Fix UBSAN undefined behaviour in flock64_to_posix_lock

locks: Fix UBSAN undefined behaviour in flock64_to_posix_lock

When the sum of fl->fl_start and l->l_len overflows,
UBSAN shows the following warning:

UBSAN: Undefined behaviour in fs/locks.c:482:29
signed integer overflow: 2 + 9223372036854775806
cannot be represented in type 'long long int'
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0xe4/0x14e lib/dump_stack.c:118
 ubsan_epilogue+0xe/0x81 lib/ubsan.c:161
 handle_overflow+0x193/0x1e2 lib/ubsan.c:192
 flock64_to_posix_lock fs/locks.c:482 [inline]
 flock_to_posix_lock+0x595/0x690 fs/locks.c:515
 fcntl_setlk+0xf3/0xa90 fs/locks.c:2262
 do_fcntl+0x456/0xf60 fs/fcntl.c:387
 __do_sys_fcntl fs/fcntl.c:483 [inline]
 __se_sys_fcntl fs/fcntl.c:468 [inline]
 __x64_sys_fcntl+0x12d/0x180 fs/fcntl.c:468
 do_syscall_64+0xc8/0x5a0 arch/x86/entry/common.c:293
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fix it by parenthesizing 'l->l_len - 1'.

Signed-off-by: Luo Meng <luomeng12@huawei.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>