osd: add SETALLOCHINT operation

This is primarily for librbd/krbd's benefit and is supposed to combat
fragmentation:

"... knowing that rbd images have a 4m size, librbd can pass a hint
that will let the osd do the xfs allocation size ioctl on new files so
that they are allocated in 1m or 4m chunks.  We've seen cases where
users with rbd workloads have very high levels of fragmentation in xfs
and this would mitigate that and probably have a pretty nice
performance benefit."

SETALLOCHINT is considered advisory, so our backwards compatibility
mechanism here is to set FAILOK flag for all SETALLOCHINT ops.

xfs is hooked up in the subsequent commits.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
FileStore: refactor FS detection checks a bit

Refactor FS detection checks in FileStore::_detect_fs() so that they
look the same as the ones in FileStore::mkfs().  This is in preparation
for adding XfsFileStoreBackend class.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
FileStore: introduce XfsFileStoreBackend class

Introduce XfsFileStoreBackend class, currently the only filestore
backend implementing SETALLOCHINT op.  This commit adds a build-time
dependency on libxfs as xfs-specific ioctl (XFS_IOC_FSSETXATTR /
XFS_XFLAG_EXTSIZE) is used to implement the new set_alloc_hint()
method.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
FileStore: add option to cap alloc hint size

Add a new config option, filestore_max_alloc_hint_size, to cap
SETALLOCHINT hint size.  The unit is a byte, the default value is
1 megabyte.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
librbd: prefix rbd writes with CEPH_OSD_OP_SETALLOCHINT osd op

In an effort to reduce fragmentation, prefix every rbd write with
a CEPH_OSD_OP_SETALLOCHINT osd op with an expected_write_size value set
to the object size (1 << order).  Backwards compatibility is taken care
of on the osd side.

"The CEPH_OSD_OP_SETALLOCHINT hint is durable, in that it's enough to
do it once.  The reason every rbd write is prefixed is that rbd doesn't
explicitly create objects and relies on writes creating them
implicitly, so there is no place to stick a single hint op into.  To
get around that we decided to prefix every rbd write with a hint (just
like write and setattr ops, hint op will create an object implicitly if
it doesn't exist)."

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>