os/FileStore: verify kernel is new enough before using extsize ioctl

os/FileStore: verify kernel is new enough before using extsize ioctl

Old kernels have an XFS bug that exposes uninitialized data when the
extsize hint is set and only partially written.  This is fixed by Linux
commit aff3a9edb7080f69f07fe76a8bd089b3dfa4cb5d, documented in XFS bug
http://oss.sgi.com/bugzilla/show_bug.cgi?id=874, and tested by XFS
test xfs/229 to prevent regressions.

Notably the original bug affects kernel 3.2, which is widely deployed with
ubuntu precise 12.04.

Backport: giant, firefly
Signed-off-by: Sage Weil <sage@redhat.com>