gfs2: Prevent inode creation race (2)

gfs2: Prevent inode creation race (2)

In gfs2_try_evict(), we try grabbing the inode to evict, we try to evict
it, and then we try grabbing it again to see if it still exists.  There
is no guarantee that we will end up with the same inode both times; the
inode validity check that commit ffd1cf0443a2 ("gfs2: Prevent inode
creation race") added to the first grab is actually needed both times.

(To avoid code duplication, add a grab_existing_inode() helper.)

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