test_librbd.cc: fix compiler warning, cast to size_t

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
BtrfsFileStoreBackend.cc: fix string overflow

Use strncpy() instead of strcpy and make sure the result
is '\0' terminated.

Fix for:

CID 1063700 (#1 of 1): Copy into fixed size buffer (STRING_OVERFLOW)
 fixed_size_dest: You might overrun the 4088 byte fixed-size string
 vol_args.name by copying the return value of c_str without checking
 the length.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Merge pull request #2623 from ceph/wip-coverity-20141002

build and coverity fixes

Reviewed-by: Sage Weil <sage@redhat.com>