fs/pipe: remove redundant initialization of pointer buf

The pointer buf is being initializated with a value that is never read,
it is being re-assigned later on at the pointer where it is being used.
The initialization is redundant and can be removed. Cleans up clang scan
build warning:

fs/pipe.c:492:24: warning: Value stored to 'buf' during its
initialization is never read [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Message-Id: <20230818144556.1208082-1-colin.i.king@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>