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When setting and clearing a delalloc range, at btrfs_set_delalloc_extent()
and btrfs_clear_delalloc_extent(), we are adding/removing the inode
to/from the root's list of delalloc inodes while under the protection of
the inode's lock. This however is not needed, we can add and remove the
inode to the root's list without holding the inode's lock because here
we are under the protection of the io tree's lock, reducing the size of
the critical section delimited by the inode's lock. The inode's lock is
used in many other places such as when finishing an ordered extent (when
calling btrfs_update_inode_bytes() or btrfs_delalloc_release_metadata(),
or decreasing the number of outstanding extents) or when reserving space
when doing a buffered or direct IO write (calls to functions from
delalloc-space.c).

So move the inode add/remove operations to the root's list of delalloc
inodes to outside the critical section delimited by the inode's lock.
This also allows us to get rid of the BTRFS_INODE_IN_DELALLOC_LIST flag
since we can rely on the inode's delalloc bytes counter to determine if
the inode is or is not in the list.

The following fio based test, that exercises IO to multiple files in the
same subvolume, was used to test:

   $ cat test.sh
   #!/bin/bash

   DEV=/dev/nullb0
   MNT=/mnt/nullb0
   MOUNT_OPTIONS="-o ssd"

   mkfs.btrfs -f $DEV &> /dev/null
   mount $MOUNT_OPTIONS $DEV $MNT

   fio --direct=0 --ioengine=sync --thread --directory=$MNT \
       --invalidate=1 --group_reporting=1 \
       --new_group --rw=randwrite --size=50m --numjobs=200 \
       --bs=4k --fsync_on_close=0 --fallocate=none --end_fsync=0 \
       --name=foo --filename_format=FioWorkloads.\$jobnum

   umount $MNT

The test was run on a non-debug kernel (Debian's default kernel config)
against a 16G null block device.

Result before this patch:

   WRITE: bw=81.9MiB/s (85.9MB/s), 81.9MiB/s-81.9MiB/s (85.9MB/s-85.9MB/s), io=9.77GiB (10.5GB), run=122136-122136msec

Result after this patch:

   WRITE: bw=86.8MiB/s (91.0MB/s), 86.8MiB/s-86.8MiB/s (91.0MB/s-91.0MB/s), io=9.77GiB (10.5GB), run=115180-115180msec
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>