fs/pipe: remove duplicate "offset" initializer

This code duplication was introduced by commit a194dfe6 ("pipe:
Rearrange sequence in pipe_write() to preallocate slot"), but since
the pipe's mutex is locked, nobody else can modify the value
meanwhile.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Message-Id: <20230919074045.1066796-1-max.kellermann@ionos.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>