arm64: mte: Handle fatal signal in reserve_metadata_storage()

As long as a fatal signal is pending, alloc_contig_range() will fail with
-EINTR. This makes it impossible for tag storage allocation to succeed, and
the page allocator will print an OOM splat.

The process is going to be killed, so return 0 (success) from
reserve_metadata_storage() to allow the page allocator to make progress.
set_pte_at() will map it with PAGE_METADATA_NONE and subsequent accesses
from different threads will trap until the signal is delivered.