[PATCHv2 fs/bfs 0/2] bfs: fix null-ptr-deref and possible warning in bfs_move_block() func

[kovalev]

https://syzkaller.appspot.com/bug?extid=d98fd19acd08b36ff422

[PATCHv2 fs/bfs 1/2] bfs: prevent null pointer dereference in bfs_move_block()
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v2: corrected the commit message and explicitly initialized
the return variable with zero (Markus Elfring)
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[PATCHv2 fs/bfs 2/2] bfs: add buffer_uptodate check before mark_buffer_dirty
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v2: renamed the return variable  err -> ret (Markus Elfring)
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