xfs: don't try redundant allocations in xfs_rtallocate_extent_near()

xfs_rtallocate_extent_near() tries to find a free extent as close to a
target bitmap block given by bbno as possible, which may be before or
after bbno. Searching backwards has a complication: the realtime summary
accounts for free space _starting_ in a bitmap block, but not straddling
or ending in a bitmap block. So, when the negative search finds a free
extent in the realtime summary, in order to end up closer to the target,
it looks for the end of the free extent. For example, if bbno - 2 has a
free extent, then it will check bbno - 1, then bbno - 2. But then if
bbno - 3 has a free extent, it will check bbno - 1 again, then bbno - 2
again, and then bbno - 3. This results in a quadratic loop, which is
completely pointless since the repeated checks won't find anything new.

Fix it by remembering where we last checked up to and continue from
there. This also obviates the need for a check of the realtime summary.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>