btrfs: always pass __GFP_NOWARN from add_ra_bio_pages()

btrfs: always pass __GFP_NOWARN from add_ra_bio_pages()

A build workload newly prints order-0 allocation failures on 7.1-rc1:

    sh: page allocation failure: order:0
    mode:0x14084a(__GFP_HIGHMEM|__GFP_MOVABLE|__GFP_IO|__GFP_KSWAPD_RECLAIM|
                  __GFP_COMP|__GFP_HARDWALL)
    CPU: 27 UID: 1000 PID: 855540 Comm: sh Not tainted 7.1.0-rc1-llvm-00058-gdca922e019dd #1 PREEMPTLAZY
    Call Trace:
     <TASK>
     dump_stack_lvl+0x50/0x70
     warn_alloc+0xeb/0x100
     __alloc_pages_slowpath+0x567/0x5a0
     ? filemap_get_entry+0x11a/0x140
     __alloc_frozen_pages_noprof+0x249/0x2d0
     alloc_pages_mpol+0xe4/0x180
     folio_alloc_noprof+0x80/0xa0
     add_ra_bio_pages+0x13c/0x4b0
     btrfs_submit_compressed_read+0x229/0x300
     submit_one_bio+0x9e/0xe0
     btrfs_readahead+0x185/0x1a0
     [...]

    (lldb) source list -a add_ra_bio_pages+0x13c
    .../vmlinux.unstripped add_ra_bio_pages + 316 at .../fs/btrfs/compression.c:454:8
       451
       452                  folio = filemap_alloc_folio(mapping_gfp_constraint(mapping, constraint_gfp),
       453                                              0, NULL);
    -> 454                  if (!folio)
       455                          break;

I can reproduce this consistently by running a memory hog concurrently
with a buffered writer on a machine with a very large amount of swap.

Commit 7ae37b2c94ed ("btrfs: prevent direct reclaim during compressed
readahead") clearly intended to suppress these warnings. But because the
mask set in the address_space with mapping_set_gfp_mask() doesn't include
__GFP_NOWARN, mapping_gfp_constraint() removes it from constraint_gfp
before it is passed to filemap_alloc_folio().

Fix by refactoring the code to add __GFP_NOWARN after the call to
mapping_gfp_constraint().

Fixes: 7ae37b2c94ed ("btrfs: prevent direct reclaim during compressed readahead")
Signed-off-by: Calvin Owens <calvin@wbinvd.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>