GFS2: Prevent double brelse in gfs2_meta_indirect_buffer

GFS2: Prevent double brelse in gfs2_meta_indirect_buffer

Before this patch, problems reading in indirect buffers would send
an IO error back to the caller, and release the buffer_head with
brelse() in function gfs2_meta_indirect_buffer, however, it would
still return the address of the buffer_head it released. After the
error was discovered, function gfs2_block_map would call function
release_metapath to free all buffers. That checked:
if (mp->mp_bh[i] == NULL) but since the value was set after the
error, it was non-zero, so brelse was called a second time. This
resulted in the following error:

kernel: WARNING: at fs/buffer.c:1224 __brelse+0x3a/0x40() (Tainted: G        W  -- ------------   )
kernel: Hardware name: RHEV Hypervisor
kernel: VFS: brelse: Trying to free free buffer

This patch changes gfs2_meta_indirect_buffer so it only sets
the buffer_head pointer in cases where it isn't released.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Acked-by: Steven Whitehouse <swhiteho@redhat.com>