GFS2: Remove test which is always true

Since gfs2_inplace_reserve() is always called with a valid
alloc parms structure, there is no need to test for this
within the function itself - and in any case, after we've
all ready dereferenced it anyway.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>