CVE-2025-68356: gfs2: Prevent recursive memory reclaim

In the Linux kernel, the following vulnerability has been resolved:

gfs2: Prevent recursive memory reclaim

Function newinode() returns a new inode with inode->imapping->gfpmask set to GFPHIGHUSERMOVABLE. This value includes the GFPFS flag, so allocations in that address space can recurse into filesystem memory reclaim. We don't want that to happen because it can consume a significant amount of stack memory.

Worse than that is that it can also deadlock: for example, in several places, gfs2unstuffdinode() is called inside filesystem transactions. This calls filemapgrabfolio(), which can allocate a new folio, which can trigger memory reclaim. If memory reclaim recurses into the filesystem and starts another transaction, a deadlock will ensue.

To fix these kinds of problems, prevent memory reclaim from recursing into filesystem code by making sure that the gfpmask of inode address spaces doesn't include GFPFS.

The "meta" and resource group address spaces were already using GFPNOFS as their gfpmask (which doesn't include GFPFS). The default value of GFPHIGHUSERMOVABLE is less restrictive than GFPNOFS, though. To avoid being overly limiting, use the default value and only knock off the GFPFS flag. I'm not sure if this will actually make a difference, but it also shouldn't hurt.

This patch is loosely based on commit ad22c7a043c2 ("xfs: prevent stack overflows from page cache allocation").

Fixes xfstest generic/273.