GFS2: Fix symlink creation race

In certain cases symlinks can appear to have zero size if a lookup
on the inode occurs within a certain (very short) time after the
symlink has been created. The symlink is correctly created on disk
but appears to have zero size when stat()ed. This patch closes the
race and prevents incorrect sizes appearing.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>