orangefs: remove redundant assignment to variable buffer_index

The variable buffer_index is assigned a value that is never read,
it is assigned just before the function returns. The assignment is
redundant and can be removed.

Cleans up clang scan build warning:
fs/orangefs/file.c:276:3: warning: Value stored to 'buffer_index'
is never read [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>