When the system is close-to-OOM, fsync() may fail due to -ENOMEM because
xfs_log_reserve() is using KM_MAYFAIL. It is a bad thing to fail writeback
operation due to user-triggerable OOM condition. Since we are not using
KM_MAYFAIL at xfs_trans_alloc() before calling xfs_log_reserve(), let's
use the same flags at xfs_log_reserve().

  oom-torture: page allocation failure: order:0, mode:0x46c40(GFP_NOFS|__GFP_NOWARN|__GFP_RETRY_MAYFAIL|__GFP_COMP), nodemask=(null)
  CPU: 7 PID: 1662 Comm: oom-torture Kdump: loaded Not tainted 5.3.0-rc2+ #925
  Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00
  Call Trace:
   dump_stack+0x67/0x95
   warn_alloc+0xa9/0x140
   __alloc_pages_slowpath+0x9a8/0xbce
   __alloc_pages_nodemask+0x372/0x3b0
   alloc_slab_page+0x3a/0x8d0
   new_slab+0x330/0x420
   ___slab_alloc.constprop.94+0x879/0xb00
   __slab_alloc.isra.89.constprop.93+0x43/0x6f
   kmem_cache_alloc+0x331/0x390
   kmem_zone_alloc+0x9f/0x110 [xfs]
   kmem_zone_alloc+0x9f/0x110 [xfs]
   xlog_ticket_alloc+0x33/0xd0 [xfs]
   xfs_log_reserve+0xb4/0x410 [xfs]
   xfs_trans_reserve+0x1d1/0x2b0 [xfs]
   xfs_trans_alloc+0xc9/0x250 [xfs]
   xfs_setfilesize_trans_alloc.isra.27+0x44/0xc0 [xfs]
   xfs_submit_ioend.isra.28+0xa5/0x180 [xfs]
   xfs_vm_writepages+0x76/0xa0 [xfs]
   do_writepages+0x17/0x80
   __filemap_fdatawrite_range+0xc1/0xf0
   file_write_and_wait_range+0x53/0xa0
   xfs_file_fsync+0x87/0x290 [xfs]
   vfs_fsync_range+0x37/0x80
   do_fsync+0x38/0x60
   __x64_sys_fsync+0xf/0x20
   do_syscall_64+0x4a/0x1c0
   entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fixes: eb01c9cd ("[XFS] Remove the xlog_ticket allocator")
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>