mm: shmem: fix too little space for tmpfs only fallback 4KB

mm: shmem: fix too little space for tmpfs only fallback 4KB

When the system memory is sufficient, allocating memory is always
successful, but when tmpfs size is low (e.g.  1MB), it falls back directly
from 2MB to 4KB, and other small granularity (8KB ~ 1024KB) will not be
tried.

Therefore add check whether the remaining space of tmpfs is sufficient for
allocation.  If there is too little space left, try smaller large folio.

Link: https://lkml.kernel.org/r/20250908123128.900254-1-vernon2gm@gmail.com
Fixes: acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs")
Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Daniel Gomez <da.gomez@samsung.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>