gfs2: Remove redundant check from gfs2_glock_dq

In function gfs2_glock_dq, it checks to see if this is the fast path.
Before this patch, it checked both "find_first_holder(gl) == NULL" and
list_empty(&gl->gl_holders), which is redundant. If gl_holders is empty
then find_first_holder must return NULL. This patch removes the
redundancy.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>