FileStore: detect XFS properly

We were only setting m_fs_type = FS_TYPE_XFS if
m_filestore_replica_fadvise was also set -- presumably
the bug fix accidentally blocked off too much of the code type. This
resulted in our xattr counts always being set too low: the store
is mounted (and thus does _detectfs) twice; once in as part of the
not-as-conditional-as-it-looks convertfs in ceph_osd.cc, and once
as part of OSD::init().

Reported-by: Sushma Gurram <Sushma.Gurram@sandisk.com>
Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>