xfs: load rtbitmap and rtsummary extent mapping btrees at mount time

It turns out that GETFSMAP and online fsck have had a bug for years due
to their use of ILOCK_SHARED to coordinate their linear scans of the
realtime bitmap.  If the bitmap file's data fork happens to be in BTREE
format and the scan occurs immediately after mounting, the incore bmbt
will not be populated, leading to ASSERTs tripping over the incorrect
inode state.  Because the bitmap scans always lock bitmap buffers in
increasing order of file offset, it is appropriate for these two callers
to take a shared ILOCK to improve scalability.

To fix this problem, load both data and attr fork state into memory when
mounting the realtime inodes.  Realtime metadata files aren't supposed
to have an attr fork so the second step is likely a nop.

On most filesystems this is unlikely since the rtbitmap data fork is
usually in extents format, but it's possible to craft a filesystem that
will by fragmenting the free space in the data section and growfsing the
rt section.

Fixes: 4c934c7d ("xfs: report realtime space information via the rtbitmap")
Also-Fixes: 46d9bfb5 ("xfs: cross-reference the realtime bitmap")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>