XfsFileStoreBackend: nuke redundant goto in set_extsize()

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
XfsFileStoreBackend: call ioctl(XFS_IOC_FSSETXATTR) less often

No need to call ioctl(XFS_IOC_FSSETXATTR) if extsize is already set to
the value we want or if any extents are allocated - XFS will refuse to
change extsize in that's the case.

Fixes: #8241
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #1930 from ceph/wip-set-extsize-fix

Wip set extsize fix

Backport: firefly
Reviewed-by: Sage Weil <sage@inktank.com>