[SRU Impish,Hirsute,HWE-5.11,OEM-5.10,Focal,Bionic] CVE-2021-4155

[Thadeu Lima de Souza Cascardo]

[Impact]
Unprivileged users may leak filesystem data.

[Fix]
Always use XFS_BMAPI_PREALLOC when calling xfs_alloc_file_space.
Focal and Bionic backports simply did that as xfs_ioc_space has changed since
then.

[Test case]
Doing allocations with XFS_IOC_ALLOCSP ioctl and checking for a leak.
It was reproduced with unpatched kernels.

[Potential regression]
XFS users may regress, specially when using XFS specific ioctls/tools.