failed to check free-space and get inodes information in btrfs

[Ezekiel Lee]

My file system is btrfs, and I found that an exception occurs when poste.io regularly executes the check free-space task every day. And the system status interface on web cannot display the inodes information, I guess these two problems are both actually related to the file system.

[SH]

fix #899 handle situations where inodes not available (ex. brtfs)

→ <<cset d29fc201e2ba>>

[Alexander Simonov]

Issue #911 was marked as a duplicate of this issue.