os/FileStore: fix compile when sync_file_range is missing;

If sync_file_range is not present, we always close inline, and flush
via fdatasync(2).

Fixes compile on ancient platforms like RHEL5.8.

Backport: bobtail
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
os/FileStore: only flush inline if write is sufficiently large

Honor filestore_flush_min in the inline flush case.

Backport: bobtail
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>