xfs: fix compile error when libxfs header used in C++ code

xfs_ialloc.h:102: error: expected ',' or '...' before 'delete'

Simple parameter rename, no changes to behaviour.

Signed-off-by: Roger Willcocks <roger@filmlight.ltd.uk>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>