ANDROID: fix isolate_migratepages_range return value

When [1] was cherry-picked from 5.10 into 5.15 kernel, it modified
the variable used to store isolate_migratepages_block() return value
like it was done in 5.10. However in 5.15 the variable used to store
the return value is different. As a result, failure to isolate a block
is not reported back to the caller. Fix by restoring the original
code and using the right variable to store the return value.

[1] ANDROID: mm: do not allow file-backed pages from CMA

Bug: 326556976
Change-Id: I06900eb43de356584ff63acfe6e994f11610b494
Signed-off-by: Suren Baghdasaryan <surenb@google.com>