btrfs: make FS_INFO ioctl available to anyone

This ioctl provides basic info about the filesystem that can be obtained
in other ways (eg. sysfs), there's no reason to restrict it to
CAP_SYSADMIN.

Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>