monitordisk.py: disable the inode checking for some fs

There is an error when use disk monitor on btrfs:
WARNING: The free inode of rootfs is running low (0.000K left)
ERROR: Immediately abort since the disk space monitor action is "ABORT"!

This is beucase some fs formats' statvfs.f_files (inodes) is zero, thus
the statvfs.f_favail (free inodes) is zero, too, this a feature of the
fs, we disable the inode checking for such a fs.

[YOCTO #3609]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>