mremap: check if it's possible to split original vma

If original VMA can't be split at the desired address, do_munmap() will
fail and leave both new-copied VMA and old VMA.  De-facto it's
MREMAP_DONTUNMAP behaviour, which is unexpected.

Currently, it may fail such way for hugetlbfs and dax device mappings.

Minimize such unpleasant situations to OOM by checking .may_split() before
attempting to create a VMA copy.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Dmitry Safonov <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Brian Geffon <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Dan Carpenter <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Dave Jiang <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Ralph Campbell <[email protected]>
Cc: Russell King <[email protected]>
Cc: Thomas Bogendoerfer <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vishal Verma <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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