ARC: mm: do_page_fault refactor #8: release mmap_sem sooner

In case of successful page fault handling, this patch releases mmap_sem
before updating the perf stat event for major/minor faults. So even
though the contention reduction is NOT super high, it is still an
improvement.

There's an additional code size improvement as we only have 2 up_read()
calls now.

Note to myself:
--------------

1. Given the way it is done, we are forced to move @bad_area label earlier
causing the various "goto bad_area" cases to hit perf stat code.

- PERF_COUNT_SW_PAGE_FAULTS is NOW updated for access errors which is what
arm/arm64 seem to be doing as well (with slightly different code)
- PERF_COUNT_SW_PAGE_FAULTS_{MAJ,MIN} must NOT be updated for the
error case which is guarded by now setting @fault initial value
to VM_FAULT_ERROR which serves both cases when handle_mm_fault()
returns error or is not called at all.

2. arm/arm64 use two homebrew fault flags VM_FAULT_BAD{MAP,MAPACCESS}
which I was inclined to add too but seems not needed for ARC

- given that we have everything is 1 function we can still use goto
- we setup si_code at the right place (arm* do that in the end)
- we init fault already to error value which guards entry into perf
stats event update

Cc: Peter Zijlstra <[e-mail]>
Signed-off-by: Vineet Gupta <[e-mail]>

arch/arc/mm/fault.c

···
 		}
 	}
 
+bad_area:
+	up_read(&mm->mmap_sem);
+
 	/*
 	 * Major/minor page fault accounting
 	 * (in case of retry we only land here once)
···
 		}
 
 		/* Normal return path: fault Handled Gracefully */
-		up_read(&mm->mmap_sem);
 		return;
 	}
-
-bad_area:
-	up_read(&mm->mmap_sem);
 
 	if (!user_mode(regs))
 		goto no_context;