linux-user/syscall.c: drop 64 suffix from flock64 &Co

linux-user/syscall.c: drop 64 suffix from flock64 &Co

Since we are always building with LFS enabled, in particular
with -D_FILE_OFFSET_BITS=64, we should always have struct flock
mapped to the 64bit variant (with off64_t), and F_GETLK mapped
to F_GETLK64 etc, automatically.

So there should be no need to explicitly use the "64" suffix
for these things anymore.

Also fix a misleading comment near safe_fcntl telling us to
always use flock64 (since v2.6.0-1311-g435da5e7092a "linux-user:
Use safe_syscall wrapper for fcntl").

Reference: https://gitlab.com/qemu-project/qemu/-/issues/2215
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>