The linux-next build bot found a three of warnings, this addresses all of them.

 * non-ANSI function declaration of function 'ceph_fscache_register' and
   'ceph_fscache_unregister'
 * symbol 'ceph_cache_netfs' was not declared, now it's extern in the header.
 * warning: "pr_fmt" redefined
Signed-off-by: NMilosz Tanski <milosz@adfin.com>

Previously we would always try to enqueue work even if the filesystem is not
mounted with fscache enabled (or the file has no cookie). In the case of the
filesystem mouned nofsc (but with fscache compiled in) this would lead to a
crash.
Signed-off-by: NMilosz Tanski <milosz@adfin.com>

Previously ceph_readpage_to_fscache did not call if page was marked as cached
before calling fscache_write_page resulting in a BUG inside of fscache.

FS-Cache: Assertion failed
------------[ cut here ]------------
kernel BUG at fs/fscache/page.c:874!
invalid opcode: 0000 [#1] SMP
Call Trace:
 [<ffffffffa02e6566>] __ceph_readpage_to_fscache+0x66/0x80 [ceph]
 [<ffffffffa02caf84>] readpage_nounlock+0x124/0x210 [ceph]
 [<ffffffffa02cb08d>] ceph_readpage+0x1d/0x40 [ceph]
 [<ffffffff81126db6>] generic_file_aio_read+0x1f6/0x700
 [<ffffffffa02c6fcc>] ceph_aio_read+0x5fc/0xab0 [ceph]
Signed-off-by: NMilosz Tanski <milosz@adfin.com>
Signed-off-by: NSage Weil <sage@inktank.com>

Adding support for fscache to the Ceph filesystem. This would bring it to on
par with some of the other network filesystems in Linux (like NFS, AFS, etc...)

In order to mount the filesystem with fscache the 'fsc' mount option must be
passed.
Signed-off-by: NMilosz Tanski <milosz@adfin.com>
Signed-off-by: NSage Weil <sage@inktank.com>