bcachefs: Fix an unhandled transaction restart

bcachefs: Fix an unhandled transaction restart

__bch2_read() -> __bch2_read_extent() -> bch2_bucket_io_time_reset() may
cause a transaction restart, which we don't return an error for because
it doesn't prevent us from making forward progress on the read we're
submitting.

Instead, change __bch2_read() and bchfs_read() to check for transaction
restarts.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>