Linux 4.9: libceph: use BUG() instead of BUG_ON(1)

[Heinrich Schuchardt]

Dear Greg,

could you, please, consider the following patch for the next Linux 4.9
release to avoid a build failure with gcc 4.9:

d24cdcd3e40a6825135498e11c20c7976b9bf545
libceph: use BUG() instead of BUG_ON(1)

Best regards

Heinrich Schuchardt