mm: reclaim mustn't enter FS for SWP_FS_OPS swap-space

If swap-out is using filesystem operations (SWP_FS_OPS), then it is not
safe to enter the FS for reclaim.  So only down-grade the requirement for
swap pages to __GFP_IO after checking that SWP_FS_OPS are not being used.

This makes the calculation of "may_enter_fs" slightly more complex, so
move it into a separate function.  with that done, there is little value
in maintaining the bool variable any more.  So replace the may_enter_fs
variable with a may_enter_fs() function.  This removes any risk for the
variable becoming out-of-date.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: NeilBrown <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Tested-by: David Howells <[email protected]>
Tested-by: Geert Uytterhoeven <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Trond Myklebust <[email protected]>
Cc: Miaohe Lin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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