os/bluestore: check return value of _open_bluefs.

os/bluestore: check return value of _open_bluefs.

With an non-existing device as db, it met the following error messages:

sceph7: 2018-12-11 04:52:28.117 7efe74b1f600 -1 bluestore(/var/lib/ceph/mnt/osd-device-0-data) _read_fsid unparsable uuid
sceph7: 2018-12-11 04:52:28.121 7efe74b1f600 -1 bluestore(/var/lib/ceph/mnt/osd-device-0-data) _open_bluefs /var/lib/ceph/mnt/osd-device-0-data/block.db symlink exists but target unusable: (2) No such file or directory
sceph7: *** Caught signal (Segmentation fault) **
sceph7:  in thread 7efe74b1f600 thread_name:ceph-osd
sceph7:  ceph version 14.0.1-1121-gfbea0e658a (fbea0e658a94c83936d2f78c42b5a8650b9700c4) nautilus (dev)
sceph7:  1: (()+0x29e7ebc) [0x564a473f8ebc]
sceph7:  2: (()+0x12890) [0x7efe7235f890]
sceph7:  3: (__pthread_mutex_trylock()+0x14) [0x7efe72357244]
sceph7:  4: (ceph::mutex_debug_detail::mutex_debug_impl<false>::try_lock_impl()+0x1f) [0x564a46ae5091]
sceph7:  5: (ceph::mutex_debug_detail::mutex_debug_impl<false>::try_lock(bool)+0x1d) [0x564a46abd3bf]
sceph7:  6: (ceph::mutex_debug_detail::mutex_debug_impl<false>::lock(bool)+0x49) [0x564a46a9b20b]
sceph7:  7: (std::lock_guard<ceph::mutex_debug_detail::mutex_debug_impl<false> >::lock_guard(ceph::mutex_debug_detail::mutex_debug_impl<false>&)+0x2f) [0x564a46a98515]
sceph7:  8: (BlueFS::mkdir(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)+0x45) [0x564a4739a7cb]
sceph7:  9: (BlueRocksEnv::CreateDir(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)+0x3a) [0x564a473b691a]
sceph7:  10: (BlueStore::_open_db(bool, bool)+0x11b3) [0x564a471ec2b7]
sceph7:  11: (BlueStore::mkfs()+0x149a) [0x564a471f4e00]
sceph7:  12: (OSD::mkfs(CephContext*, ObjectStore*, uuid_d, int)+0xc9) [0x564a46a07c2b]
sceph7:  13: (main()+0x1bf0) [0x564a469d4850]
sceph7:  14: (__libc_start_main()+0xe7) [0x7efe70ff4b97]
sceph7:  15: (_start()+0x2a) [0x564a469d1c5a]
sceph7: 2018-12-11 04:52:28.137 7efe74b1f600 -1 *** Caught signal (Segmentation fault) **

To avoid this occur, it should check return value of _open_bluefs.

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>