ext4: fix possible use after free with metadata csum

commit aeb1e5d69a5be592e86a926be73efb38c55af404 upstream.

Commit fa77dcfafeaa introduces block bitmap checksum calculation into
ext4_new_inode() in the case that block group was uninitialized.
However we brelse() the bitmap buffer before we attempt to checksum it
so we have no guarantee that the buffer is still there.

Fix this by releasing the buffer after the possible checksum
computation.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Acked-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>