gfs2: Get rid of gfs2_set_nlink

Remove gfs2_set_nlink which prevents the link count of an inode from
becoming non-zero once it has reached zero.  The next commit reduces the
amount of waiting on glocks when an inode is evicted from memory.  With
that, an inode can become reallocated before all the remote-unlink
callbacks from a previous delete are processed, which causes the link
count to change from zero to non-zero.

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