fuse: fix return value of fuse_dev_write()

On 64 bit systems -- where sizeof(ssize_t) > sizeof(int) -- the following test
exposes a bug due to a non-careful return of an int or unsigned value:

implement a FUSE filesystem which sends an unsolicited notification to
the kernel with invalid opcode. The respective write to /dev/fuse
will return (1 << 32) - EINVAL with errno == 0 instead of -1 with
errno == EINVAL.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
CC: stable@kernel.org