If an inode has a very large number of extent maps, we can spend
a lot of time freeing them, which triggers a soft lockup warning.
Therefore reschedule if we need to when freeing the extent maps
while evicting the inode.

I could trigger this all the time by running xfstests/generic/299 on
a file system with the no-holes feature enabled. That test creates
an inode with 11386677 extent maps.

    $ mkfs.btrfs -f -O no-holes $TEST_DEV
    $ MKFS_OPTIONS="-O no-holes" ./check generic/299
    generic/299 382s ...
    Message from syslogd@debian-vm3 at Aug  7 10:44:29 ...
     kernel:[85304.208017] BUG: soft lockup - CPU#0 stuck for 22s! [umount:25330]
     384s
    Ran: generic/299
    Passed all 1 tests

    $ dmesg
    (...)
    [86304.300017] BUG: soft lockup - CPU#0 stuck for 23s! [umount:25330]
    (...)
    [86304.300036] Call Trace:
    [86304.300036]  [<ffffffff81698ba9>] __slab_free+0x54/0x295
    [86304.300036]  [<ffffffffa02ee9cc>] ? free_extent_map+0x5c/0xb0 [btrfs]
    [86304.300036]  [<ffffffff811a6cd2>] kmem_cache_free+0x282/0x2a0
    [86304.300036]  [<ffffffffa02ee9cc>] free_extent_map+0x5c/0xb0 [btrfs]
    [86304.300036]  [<ffffffffa02e3775>] btrfs_evict_inode+0xd5/0x660 [btrfs]
    [86304.300036]  [<ffffffff811e7c8d>] ? __inode_wait_for_writeback+0x6d/0xc0
    [86304.300036]  [<ffffffff816a389b>] ? _raw_spin_unlock+0x2b/0x40
    [86304.300036]  [<ffffffff811d8cbb>] evict+0xab/0x180
    [86304.300036]  [<ffffffff811d8dce>] dispose_list+0x3e/0x60
    [86304.300036]  [<ffffffff811d9b04>] evict_inodes+0xf4/0x110
    [86304.300036]  [<ffffffff811bd953>] generic_shutdown_super+0x53/0x110
    [86304.300036]  [<ffffffff811bdaa6>] kill_anon_super+0x16/0x30
    [86304.300036]  [<ffffffffa02a78ba>] btrfs_kill_super+0x1a/0xa0 [btrfs]
    [86304.300036]  [<ffffffff811bd3a9>] deactivate_locked_super+0x59/0x80
    [86304.300036]  [<ffffffff811be44e>] deactivate_super+0x4e/0x70
    [86304.300036]  [<ffffffff811dec14>] mntput_no_expire+0x174/0x1f0
    [86304.300036]  [<ffffffff811deab7>] ? mntput_no_expire+0x17/0x1f0
    [86304.300036]  [<ffffffff811e0517>] SyS_umount+0x97/0x100
    (...)
Signed-off-by: NFilipe Manana <fdmanana@suse.com>
Reviewed-by: NSatoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Tested-by: NSatoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: NChris Mason <clm@fb.com>