We always just try and reserve data space when we write, but if we are out of
space but have prealloc'ed extents we should still successfully write.  This
patch will try and see if we can write to prealloc'ed space and if we can go
ahead and allow the write to continue.  With this patch we now pass xfstests
generic/274.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>