um: use proper care when taking mmap lock during segfault

Segfaults can occur at times where the mmap lock cannot be taken. If
that happens the segfault handler may not be able to take the mmap lock.

Fix the code to use the same approach as most other architectures.
Unfortunately, this requires copying code from mm/memory.c and modifying
it slightly as UML does not have exception tables.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20250408074524.300153-2-benjamin@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

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